Rbm is a male infertility gene located on the Y chromosome that is expressed in the testis. To investigate the specific events of spermatogenesis in which Rbm plays a role, the precise pattern of expression of Rbm in the mouse testis was determined. An antibody was generated against the Rbm protein and used to detect a single specific band of 43 kDa in size in mouse testicular lysates. In situ hybridization, immunoblot and immunohistochemistry analyses together indicated that Rbm was expressed in spermatogonia, preleptotene spermatocytes, late leptotene to early pachytene spermatocytes but not in mid-pachytene spermatocytes or subsequent stages of differentiation, including haploid germ cells. These observations suggest that Rbm functions in early but not later stages of male germ cell development.