Effects of radioactive cesium (Cs) derived from the Fukushima Daiichi Nuclear Power Plant (FNPP) accident on the testes of the wild Japanese macaque were investigated from a morphological viewpoint. Cesium-137 (137Cs) concentration in the skeletal muscle of macaques in Fukushima Prefecture (affected group) ranged from 64.2 to 693.7 Bq/kg, which was clearly higher than that in Niigata Prefecture (unaffected group, 5.6 Bq/kg). Testicular tissue sections stained with hematoxylin and eosin were compared between the affected and unaffected groups under a light microscope. As a result, active spermatogenesis was clearly observed in the testicular tissues of both groups, and no definite differences were recognized. The distribution of vimentin in Sertoli cells was also similar in both groups. We then immunohistochemically determined the stage of the seminiferous epithelium and studied the proliferating and apoptotic spermatogenic cells and their specific cell types after type B spermatogonia at each stage in comparison between the affected and the unaffected groups. Consequently, proliferating spermatogenic cells of both macaque groups were type B spermatogonia at stages I–VI, preleptotene primary spermatocytes at stages VII and VIII, leptotene primary spermatocytes at stage IX, zygotene primary spermatocytes at stages X and XI, and pachytene primary spermatocytes at stages XII–VII. Meanwhile, apoptotic spermatogenic cells were type B spermatogonia at stages I–VI, preleptotene primary spermatocytes at stages VII and VIII, and leptotene primary spermatocytes at stage IX in both groups, though the spermatogenic cells above were not always cleaved caspase-3-positive apoptotic cells. Thus, no concrete differences were confirmed between the affected and the unaffected macaque groups in either proliferating or apoptotic spermatogenic cells at any stage of the seminiferous epithelium or in any specific cell type, though 137Cs concentrations in muscles were definitely different between these two groups. In conclusion, Japanese macaques currently inhabiting the area affected by radioactive materials from the FNPP accident possess normal activity of sperm reproduction.