Hybrid sterility is thought to play an important role in the differentiation and maintenance of respective biological species. In the present study, as a first step to clarify the mechanism of hybrid sterility caused by the Hst-1 gene in mice, the male sterile hybrid between C57BL/6 and M. m. musculus was investigated by histological techniques. Many spermatogonia and primary spermatocytes were observed in the testes of the hybrid sterile mice. Neither secondary spermatocytes nor spermatids were detected throughout the sections. It was noted that some spermatocytes showed degenerations in part of the lumina of the seminiferous tubules. In the chromosomal preparations of primary sterile hybrids, the percentage statistical values for the spermatogonia/leptotene stage were increased and those for the zygotene/pachytene stage were markedly decreased. Neither round nor elongated spermatids were detected throughout the preparations. It was observed that many primary spermatocytes were degenerated. These results suggest that spermatogenesis stops at the leptotene stage and that a small number of cells progress to the next meiosis, but degenerate to die during the pachytene stage.