A sterile mouse was found to be a chromosomal mosaic with two cell lines, one containing 39 chromosomes, the other 41. Both types were present in approximately equal numbers in the bone marrow, but the 41-chromosome line alone was identified in spermatogonia and spermatocytes. Evidence from chromosome morphology at mitotic metaphase and from spermatocytes at diakinesis and metaphase I identified the two cell-lines as XO and XYY respectively. The alternative possibilities of exclusion of the XO line from the seminiferous elements by chance and by selection are discussed and the latter interpretation favoured. The possibility is raised that the Y chromosome of mammals carries genetic information essential for normal spermatogenesis, as distinct from its established function in embryonic gonadal determination.