Both sexes of Microtus oregoni are gonosomic mosaics; the male with the OY/XY constitution, the female the XX/XO. The diploid chromosome number of 17 originally reported by Matthey (1956, 1958) holds true only for male germ cells (OY) and female somatic cells (XO). The male, which develops from an XY-zygote with 18 chromosomes, produces two kinds of gametes; one with the Y, the other with no sex element; the X has been eliminated from the spermatogonia. Thus in this species the sperm with no sex element at all is female-determining; the XO-zygote develops into a female lacking a paternal X-chromo-some.