Light and electron microscopy showed active spermatogenesis in Philippine carabao bulls. A significantly high incidence of ultrastructural alterations in Sertoli and spermatogenic cells was observed in crossbred buffaloes (80%) as compared to the 23% occurrence in Philippine carabaos. The frequently observed alterations were pycnotic nuclei (20%) and intracytoplasmic vacuolations (32%) in spermatogonia and spermatocytes. Developing spermatids exhibited nuclear bleb (8%), disintegrated number membrane (28%) and some vesiculations. Sertoli cells exhibited intracavitary structures and exaggerated infoldings of the nuclear envelope (30%), nuclear bleb (16%), and intracytoplasmic vacuolations (16%). Philippine carabaos exhibited few ultrastructural alterations which were mainly intracytoplasmic vacuolations in Sertoli cells (15%) and spermatogenic cells (7.6%). Crossbred buffaloes with testicular alterations were found to have inferior semen quality compared to Philippine carabaos and crossbred buffaloes without testicular alterations.