A sterile male mouse with absence of the normal X-Y chromosome association at meiotic metaphase I is described. The X and Y chromosomes were seen together in the sex vesicle at pachytene, but they became separated soon after this stage. No secondary spermatocytes were found in air-dried testicular chromosome preparations, and no spermatozoa were seen in the epididymes. The mitotic chromosomes of the sterile mouse were normal. The father and a fertile brother of the sterile male had a higher-than-normal proportion of spermatocytes with failure of X-Y chromosome association at metaphase I. The ratios of the length of the X to the length of the Y chromosome in the sterile male, the fertile brother, and a chromosomally normal male of the same stock were not significantly different. Seventeen other tested sibs of the sterile male proved to be fully fertile. The mating in which the sterile male was found gave small litters with an excess of males (31 males:14 females). Some possible causes of the sterility are discussed.