The aim of this study was to investigate the spermatogenesis in ksr2(-/-) mice. Spermatogenesis in 12-15 week-old C57BL/6 wt and ksr2(-/-) mice was observed in testicular tissue and epididymal sperm by light and transmission electron microscopy. The reproductive capacity of male ksr2(-/-) mice was strongly impaired. Concentration, morphology and motility of epididymal spermatozoa were altered in ksr2(-/-) mice. In seminiferous tubules from ksr2(-/-) mice, all stages of spermatogenetic process were represented; spermatids displayed defects concerning nuclear and acrosomal shape and periaxonemal structures of the tail; detached head and spermatozoa with an altered head-tail connection were observed; the interstitial tissue was severely disorganized, the Leydig cells have lost their connections. TEM analysis of epididymal spermatozoa confirmed the presence of such kind of alterations. We reported, for the first time, an ultrastructural study of ksr2(-/-) mice spermatogenesis. Remarkable findings regard the altered spermiogenetic process concomitant with a severe disorganization of interstitial tissue. Further studies are needed to assess the ksr2(-/-) mice hormonal status, focussing on testosterone levels since the interstitial tissue, where the Leydig cells reside, was compromised.