A cDNA encoding a protein homologous with histone H1 has been cloned from a haploid germ cell specific cDNA library. Deduced amino acid sequence (170 amino acids) showed 40% identity with histone H1 globular domain. Messenger RNA of the gene was observed exclusively in the testis, and was accumulated after post-natal day 23. Western blotting analysis showed that the protein encoded by this gene is about 19 kDa in molecular weight, and it was exclusively recovered from the nuclei of testicular germ cells. Immunohistochemical analysis showed that the protein was localized to the nuclei of round and elongating spermatids, consistent with the results of immunoblot analysis. Thus, the gene product was named Hils1 (histone H1 like protein in spermatids 1). In vitro DNA-binding experiments using DNA-cellulose mini-columns showed that Hils1 was able to bind to both double and single stranded-DNAs in a non-sequence-specific manner. These findings suggest that Hils1 may play an important role in the structural changes of spermatid nuclei, such as nuclear condensation, and gene regulation of haploid germ cell differentiation.