We have investigated the cell types in mouse testis and ovary in which the c-mos protooncogene is normally transcribed. Blot hybridization analysis of electrophoretically fractionated RNAs from testes of mice with defects in germ-cell development and from prepubertal and adult mice indicated that c-mos was transcribed during male germ-cell development. Analysis of purified populations of spermatogenic cell types detected c-mos RNA in the earliest haploid postmeiotic germ cell, the round spermatid, indicating that c-mos was expressed transiently during spermatogenesis. c-mos RNA was detected by blot hybridization in the ovaries of prepubertal mice and decreased in relative concentration following gonadotropin-stimulated proliferation of granulosa cells. These results suggested that c-mos was transcribed in oocytes and were confirmed by detection of high levels of c-mos RNA in isolated grown oocytes. Thus, c-mos is expressed in both male and female germ cells, suggesting possible roles for this protooncogene in meiosis, germ-cell development, fertilization, and early embryogenesis.