The myb gene family consists of three members, A-myb, B-myb and c-myb. The proteins encoded by these genes bind DNA in a sequence-specific manner and regulate transcription of target genes. In this communication, we report the nucleotide sequence of murine A-myb cDNA. This gene encodes a protein of 751 amino acids with an estimated molecular weight of 83 kDa. cDNA sequence analysis of multiple independent cDNA clones reveals the presence of alternatively spliced mRNAs that encode smaller proteins. Northern blot analysis of adult mouse tissue RNAs reveals A-myb expression predominantly in the testis, with very low levels of expression in the ovaries, spleen and brain. In situ hybridization analysis of adult mouse testis shows that this gene is expressed at high levels in type A spermatogonia (stem cells), and preleptotene and pachytene spermatocytes, with concomitant down-regulation of expression upon terminal differentiation of these cells into mature spermatozoa. This pattern of expression suggests that A-myb is involved in the control of proliferation and differentiation of spermatogonia. This potential function of A-myb in spermatogenesis is reminiscent of the role of c-myb in hematopoiesis.