We recently reported the cloning and sequencing of the mouse A-myb proto-oncogene cDNA and the abundant expression of this mRNA primarily in the testis of adult mice. The A-myb mRNA is detectable by in situ hybridization specifically in the spermatogenic cells, and is downregulated during terminal differentiation. A low level of expression is observed in a few other tissues, including ovary, spleen and brain. We have extended those studies by examining A-myb and B-myb expression during testis development in the mouse. The A-myb and B-myb genes are both expressed in a cell- and stage-specific manner during testis development. The B-myb mRNA is expressed most highly in gonocytes of the fetal testis and in spermatogonia and early spermatocytes in the adult. B-myb expression decreases at day 18 post partum, coincident with the initial appearance of late pachytene spermatocytes. B-myb expression was also detectable in some interstitial cells of the fetal and adult testis. The A-myb mRNA was not detectable by in situ hybridization in fetal day 15.5 gonocytes but was detectable at a low abundance by RT-PCR in fetal and newborn mice. A-myb mRNA expression increased at post-natal day 10, when primary spermatocytes first appear. In the adult, the A-myb mRNA was expressed highly in a sub-population of spermatogonia and in primary spermatocytes, but was not detectable in spermatids. This expression of A-myb is consistent with the meiotic arrest that is observed in A-myb-deficient male mice. We conclude that B-myb may play a critical role in controlling the proliferation or differentiation of gonocytes and spermatogonia and possibly the somatic lineages as well, whereas A-myb is required for progression through the first meiotic prophase. These distinct roles for B-myb and A-myb during spermatogenesis may reflect distinct transactivation potentials of the two proteins. Further studies to determine the functions of A-myb and B-myb in the developing testis should improve our understanding of the molecular events associated with spermatogenesis and differentiation of the Sertoli and other somatic cell types of the testis.