AbstractThe localization of actin during spermiogenesis in the rat, hamster, monkey, and human was examined at the ultrastructural level using postembedding immunogold methods. Results revealed a similar pattern of actin distribution in these four species, although the staining intensity was higher in rodent spermatids than in those obtained from primates. Gold particles were first detected in the nascent subacrosomal space of round spermatids. This subacrosomal labeling extended as the acrosome spread over the nucleus during the elongation phase, remained unchanged during the first steps of the maturation phase, and disappeared completely before spermiation. Thus, using antiactin probes (present results) and other specific probes, actin appears to be a consistent component of the subacrosomal layer of spermatids during the greater part of spermiogenesis in many mammals.