c-myb has been studied intensely, but other members of the myb gene family have been largely overlooked. This paper describes the isolation and characterization of a Xenopus cDNA sequence, XAMyb, closely related to human A-myb. Xenopus provides a valuable experimental model for investigating the involvement of genes in cell proliferation, differentiation and development. Although no expression of XAMyb was detected in oocytes or eggs or during early embryogenesis, there is a low level of expression within ovarian tissue, which is down-regulated in response to hormones that stimulate oocyte maturation. However, in adult male Xenopus, virtually all A-myb expression is in the testis, particularly within the nuclei of spermatogonial cells, the mitotically proliferating early progenitors of spermatozoa. Expression falls dramatically during meiosis and is virtually undetectable during subsequent spermiogenesis. This pattern of expression of Xenopus A-myb during the preterminal differentiation/proliferation phase of germ cell development resembles the expression of c-myb in similar phases during haematopoiesis. These results suggest related roles for myb variants during the development of different stem cell-derived populations.