Hamster sperm extracts contain a polypeptide which comigrates with muscle action on polyacrylamide gel electrophoresis. On double diffusion precipitation plates, sperm extracts form a single precipitin band with an antibody to muscle actin (antiactin) and show a reaction of identity with muscle actin. Indirect immunofluorescent microscopy revealed that antiactin binds along the concave margin and equatorial segment in the acrosomal region, in the connecting piece of the neck, and in the principal piece of the tail. These results are evidence that hamster sperm contain actin. The possible significance of these observations in fertilization is considered.