Actin-related proteins (Arps) have been reported to be localized in the cell nucleus, and implicated in the regulation of chromatin and nuclear structure, as well as being involved in cytoplasmic functions. We demonstrate here that mouse ArpM1, which closely resembles the conventional actin, is expressed exclusively in the testis, particularly in haploid germ cells. ArpM1 protein first appears in the round spermatid and changes its localization dynamically in the nucleus during spermiogenesis. By co-immunoprecipitation analysis, profilin III was identified as ArpM1-interacting protein. Our findings suggest that the testis-specific profilin III-ArpM1 complex may be involved in conformational changes in the organization of the sperm-specific nucleus.