This is an appeal by plaintiff-husband in what was originally an annulment suit from the order of the trial court denying a motion to cancel all previous orders by which he was directed to pay attorney’s fees, maintenance, and court costs.
Facts: Plaintiff, aged 21, and defendant were married in Nevada on November 9, 1964. They returned to California to live. On the morning of November 13, less than four days after the marriage ceremony, plaintiff discovered that his wife had left him. They have had no contact since that date.
Plaintiff filed suit for annulment in the Superior Court of San Joaquin County, alleging that his wife had no intent to live with him at the time they were married. Defendant answered with a denial and also alleged that she was pregnant with plaintiff’s child. She asked for attorney’s fees plus support and maintenance for herself and the unborn child.
The Superior Court of San Joaquin County ordered plaintiff to pay $50 per month for the support of defendant, prenatal care, hospital expenses and counsel fees. On defendant’s motion the case was transferred to Stanislaus County, and after the child was born the court ordered plaintiff to pay hospital expenses, $50 per month child support, and to comply with the previous order of the San Joaquin County Superior Court with respect to counsel fees. The court also ordered plaintiff, defendant, and the child to submit to blood tests.
The tests were taken and, according to plaintiff’s sworn affidavit, demonstrated that plaintiff could not have fathered the child. Plaintiff moved the court to terminate all prior court orders for support, doctor and medical expenses, and submitted the results of the blood tests as evidence in support of his motion.
.^.The-. couple cohabited only three and one half to four days, and it is conceded that the child was born approximately nine months thereafter. The trial judge refused to admit the blood *247tests in evidence and denied the motion to terminate support payments.
Question: Were the blood tests admissible in evidence?
Yes. In Kusior v. Silver, 54 Cal.2d 603 [7 Cal.Rptr. 129, 354 P.2d 657], we held that blood tests were inadmissible to prove the husband had not fathered the wife’s child in circumstances where the conclusive presumption applied.1 This is so because the so-called conclusive presumption is really not a presumption but rather a rule of substantive law.
Kusior v. Silver, supra, 54 Cal.2d 603, is not applicable to the facts of the present case. The judgment in that ease was reversed because the plaintiff, who claimed that a man other than her husband fathered her child, was prejudiced by the court’s instruction defining “cohabiting.” It was essential to the plaintiff’s ease to exclude her husband, from whom she had separated, as the father. The court instructed that the conclusive presumption (Code Civ. Proc., § 1962, subd. 5, now Evid. Code, § 621) applied if the husband, who had vacated the family home but visited his wife before, during and after the period of probable conception, had “access” or “reasonable possibility of access” to his wife during the period of conception. Before the jury could apply the conclusive presumption that the husband was the father, they would have to have found cohabitation between husband and wife. Because of the erroneous definition of “cohabiting,” it was reasonably probable that the jury improperly determined that the husband was the father. In reversing, we held that “cohabiting” means to “dwell or live together as husband and wife” (pp. 612-613 [3]) and if husband and wife are not in fact residing together, an exception to the application of the conclusive presumption arises; the presumption is rebut-table and blood test evidence may be admitted to rebut it (p. 620 [14]).
It is settled that the husband is entitled to avoid the operation of the “conclusive presumption” by proof that although there was cohabitation it was impossible that the child was conceived during the period of cohabitation. (Estate of McNamara, 181 Cal. 82, 91-97 [183 P. 552, 7 A.L.R. 313] ; *248Estate of Walker, 180 Cal. 478, 491 [5] [181 P. 792] ; Estate of Marshall, 120 Cal.App.2d 747, 753 [262 P.2d 42].)
The ruling of the trial court in the present case denied plaintiff a fair opportunity to prove that defendant’s child was not conceived during the three and one half to four days the couple cohabited. If he is permitted to put in evidence the blood test results, then he could offer other evidence to prove that the child was not conceived during his brief cohabitation with his wife.
The couple were honeymooning in their apartment. Plaintiff was undoubtedly in the presence of his wife during most of this time and he should be able to account for her actions and whereabouts during the three and one half to four days of married life. Plaintiff must prove that the child was not conceived during the brief cohabitation; he must prove that it was not conceived when he was having sexual intercourse with his wife. The blood tests are evidence of this.
Blood grouping tests are scientifically reliable when used to exclude a male as a possible father. (Kusior v. Silver, supra, 54 Cal.2d at p. 617.) The adoption of the Uniform Act on Blood Tests to Determine Paternity (Stats. 1953, ch. 1426, p. 3013, now Evid. Code, §§ 890-897) is a recognition of the scientific reliability of such tests. (See also concurring opinion of Mr. Justice Fourt in Wareham v. Wareham, 195 Cal.App.2d 64, 79-83 [15 Cal.Rptr. 465] ; Sehatkin, Disputed Paternity Proceedings (3d ed. 1953) pp. 289-356 ; Witkin, Cal. Evidence (1966) § 657, p. 618.)
Kusior v. Silver, supra, 54 Cal.2d 603, correctly holds, on the facts of that ease, that the presumption of Evidence Code section 621 is not so much a conclusive presumption as it is a rule of substantive law that a husband will be treated as the father of a child born to his wife and conceived while they were cohabiting. It makes no difference whether the husband is the biological father, for the basis of the inquiry is whether he is the legal father of the child; he must be given a chance to prove that he is not the legal father by demonstrating the impossibility that the child was conceived during his cohabitation with his wife. In the instant case plaintiff was denied this opportunity.
When the issue is whether the child could possibly have been conceived during cohabitation, the evidentiary rule is " any competent evidence relevant to the question is admissible.” (Estate of Walker, supra, 180 Cal. at p. 491 [5].)
*249In the present ease the blood tests were relevant, since they prove that conception did not occur at various times during the four-day cohabitation, that is, the moments when the newlyweds were engaged in sexual intercourse.
The order is reversed.
Traynor, C. J., Peters, J., and Mosk, J., concurred.
Section 621 of the Evidence Code restates without substantive change former section 1962, subdivision 5, of the Code of Civil Procedure, as follows: “Notwithstanding any other provision of law, the issue of a wife cohabiting with her husband, who is not. impotent, is conclusively presumed to be legitimate. ’'