In the hybrid mouse strain BDF1; some 35–40 % of spermatocytes had unpaired X and Y chromosomes in stages ranging from diplotene to first meiotic metaphase. This phenomenon varied significantly from mouse to mouse. In pooled material from Swiss Albino and CF1 mice, the corresponding frequency was 5.7 %. In C57 BL/6 mice, one of the parent strains of BDFj mice, the X and Y were separate in 7.7 % of the spermatocytes. Based on the behavior of the X and Y in the BDF1 strain, it is concluded that they do not pair, rather than initially pairing and then precociously separating. The factor interfering with the pairing of the X and Y does not affect the autosomes; possibly it is an incompatibility of the two sex chromosomes, which come from different inbred line