OPINION
DOGGETT, Justice.In this mandamus proceeding, we determine that a trial court abused its discretion in failing to comply with a previous judgment of this court.
In 1986, Richard W. Lee and other police officers employed by the Houston Police Department (HPD) challenged certain ordinances of the City of Houston in an action seeking a declaratory judgment, injunctive relief, promotions with backpay, and attorney’s fees. The trial court accorded relief in a judgment which (1) declared void four City ordinances as violative of a state statute because they eliminated the civil service status of certain HPD positions;1 (2) enjoined the City from hiring civilians to fill these positions; and (3) ordered promotions with back pay and attorney’s fees to the plaintiffs. The court of appeals reversed this judgment and dissolved the injunction.2 This court reversed and remanded to the trial court “for entry of judgment in accordance with [our] opinion.” Lee v. City of Houston, 807 S.W.2d 290, 296 (Tex.1991) (Lee I).
On remand, the police officers submitted to the trial court a reformed judgment incorporating some of the language of our opinion and granting the same relief as the original judgment. Rejecting their proposal, the trial court instead issued a “Final Judgment,”3 which reads in full:
*648Pursuant to the mandate of the Texas Supreme Court issued July 1, 1991, the Court hereby finds as follows:
The Defendant’s use of unclassified employees in the nine positions at issue in this case violated Section 143.021(c) of the Texas Local Government Code. Such positions must be filled in accordance with the terms of such statute.
All releif [sic] not granted herein is denied.
The police officers return here as Relators, seeking a writ of mandamus compelling the trial court to render judgment in compliance with our earlier mandate. In Lee I, we directed the trial court to enter judgment in accordance with an opinion reversing the judgment of the court of appeals. Because its judgment does not conform to our mandate, the trial court has abused its discretion, and we conditionally grant relief. In doing so, we reject the City’s claim that mandamus is not the proper remedy. If a trial court fails to issue a judgment in accordance with our mandate, the aggrieved party may seek either a writ of prohibition or a writ of mandamus. Cherokee Water Co. v. Ross, 698 S.W.2d 363, 366 (Tex.1985, orig. proceeding); City of Orange v. Clark, 627 S.W.2d 146, 147 (Tex.1982, orig. proceeding). Mandamus will issue to ensure compliance with this court’s judgment. Gulf, Colorado & Santa Fe Ry. Co. v. City of Beaumont, 373 S.W.2d 741, 744 (Tex.1964, orig. proceeding).
Based upon its determination that “each of the Plaintiffs in this cause had attained a position on an active promotional eligibility list at the time Defendants [improperly] filled the positions which are the subject of this action with [civilians],” the trial court originally ordered that each plaintiff receive retroactive promotion as well as lost wages and benefits. Interpreting a civil service statute that lacked a clear definition of the “police officer[s]” it encompasses, we concluded unequivocally in Lee I that “the placement of unclassified employees in [nine] positions [four at the level of deputy chief and five at the level of lieutenant] contravened the Act.” 807 S.W.2d at 294-95.
The City undertook an extensive “reorganization” of its police department on January 20, 1986, by creating four new bureaus to be directed by four recently-hired, unclassified employees. The trial court originally determined that each of these bureau commanders “assumed the duties of a Deputy Chief.” We concluded in Lee I that the appointment of unclassified employees to these four positions contravened the Act. 807 S.W.2d at 295. In other words, the positions should have been filled from the promotional eligibility list for the position of Deputy Chief in existence at the time of the vacancies. See Bostick v. Owens, 423 S.W.2d 471, 472 (Tex.Civ.App—Fort Worth 1968, writ ref’d n.r.e.).4 Three of the plaintiffs — officers Milam, Simmons, and Contreras — occupied the second, third, and fourth positions on the Deputy Chief eligibility list that was in existence on January 20, 1986. Thus, each of those three officers was entitled to promotion to the position of Deputy Chief of Police effective March 21, 1986.
Two of the unclassified employees who became bureau commanders on January 20, 1986, vacated positions entailing duties which had previously been performed by captains.5 The plaintiffs asserted that these vacancies should have been filled by the appointment of officers from the eligibility list for the position of captain. We could not determine from the record, however, whether these positions must be classified. To resolve this question, the trial court should have made any *649further findings necessary to apply the test set forth in Lee I. If, by employing that test, the trial court determines that the two positions must be classified, the two officers at the top of the captain eligibility list in existence on January 20, 1986 — Officers Lee and Hendrick — should be promoted to captain effective March 21, 1986.
The promotion of four captains to deputy-chief positions on March 21, 1986, would have created four additional captain-level vacancies that should have been filled by officers at the top of the captain eligibility list then in existence. The top four officers remaining after any other promotions are entitled to promotion to the position of captain effective May 20, 1986.6
The plaintiffs also challenged the appointment of five unclassified persons to positions entailing the duties and responsibilities of lieutenants. We held in Lee I that these appointments contravened the Act. 807 S.W.2d at 295. In accord with that prior opinion, the trial court should have determined the exact dates of vacancies for these positions, as well as sergeant, and the relevant eligibility lists, awarding appropriate relief to those plaintiffs who should have been promoted to vacant positions.
It is well settled that when a city fails to fill a vacant position in accordance with the Civil Service Act, the officers or firefighters who properly should have obtained such jobs are entitled to retroactive promotion and back pay. Michna v. City of Houston, 521 S.W.2d 381, 335 (Tex.Civ. App.—Houston [1st Dist.] 1975, no writ), appeal after remand, 534 S.W.2d 728 (Tex.Civ.App.—Houston [1st Dist.] 1976, writ ref’d n.r.e.); see International Ass’n of Firefighters v. Townsend, 622 S.W.2d 562 (Tex.1981). A promotion is considered effective as of the last date that the city could lawfully have filled the vacancy; that is, sixty days from the date the vacancy was created. See Tex.Loc.Govt.Code § 143.108(b);7 Michna, 521 S.W.2d at 335; Bostick v. Owens, 423 S.W.2d 471, 472 (Tex.Civ.App.—Fort Worth 1968, writ ref’d n.r.e.).8 Additionally, a prevailing party may recover attorney’s fees. Tex.Local Gov’t Code § 143.015. The trial court judgment considered by this court in Lee I provided back pay and attorney’s fees.
We are hopeful that today's opinion will finally resolve this long delayed dispute. We are confident that Judge Downey will withdraw his July 26, 1991 judgment and, in lieu thereof, issue a judgment fully implementing the mandate of this court, as set forth here and in Lee I by identifying those plaintiffs entitled to promotion and awarding them backpay and attorney’s fees.9 The writ will issue only if he fails to comply.
. Fire Fighters’ and Police Officers' Civil Service Act, Tex.Loc.Gov’t Code §§ 143.001-143.134 (formerly Tex.Rev.Civ.Stat.Ann. art. 1269m).
. City of Houston v. Lee, 762 S.W.2d 180 (Tex. App.—Houston [1st Dist.] 1988).
.Judge Frank White entered the original judgment of the trial court; Judge Dan Downey, the second judgment.
. Insisting that no vacancies were created by the January 20, 1986 reorganization, the dissent references trial testimony of Lieutenants DeFoor and Lee. 842 S.W.2d at 654, n. 14. Neither conceded that no legal vacancy was created as a result of the reorganization; indeed, Lee’s testimony supports a contrary conclusion.
. Don Hollingsworth vacated his position as head of the Houston Crime Information Center (now the Computer Services Division) to become Bureau Commander of the Management Information Bureau. Jack Seitzinger vacated his position as head of the Police Training Academy to become Commander of the Management Information Bureau.
. The top five positions on the list in existence from April 11, 1985, to April 11, 1986, were held by plaintiffs Lee, Hendrick, Yates, DeFoor, and Merrill. Although the list expired before the sixty-day deadline would have passed, it nonetheless determines eligibility for the March 21 vacancy. See Bostick v. Owens, 423 S.W.2d at 472.
. This sixty-day deadline applies to all Texas municipalities through section 143.108(b) (for cities with population of 1.5 million or more) and section 143.036(e) (for other municipalities).
. When Michna and Bostick were decided, the predecessor to section 143.108 allowed ninety days for the filling of vacancies. Act of July 5, 1949, ch. 572, § 2, 1949 Tex.Gen.Laws 1114, 1115 (amending Tex.Rev.Civ.Stat. Ann. art. 1269m, § 8). In 1979, however, the Legislature shortened the period to sixty days. Act of May 28, 1979, ch. 753, § 3, 1979 Tex.Gen.Laws 1856, 1857.
.Although their proposed judgment on remand sought to reinstate a permanent injunction that the City cease illegal civilian appointments, the Relators have indicated in both their Petition for Mandamus and in oral argument their belief that the City will demonstrate future good faith compliance with Lee I. Assuming this agreed change in circumstance, the trial court did not abuse its discretion in denying permanent in-junctive relief.