We report here the molecular cloning and characterization of a human orthologue of oppo 1, a mouse gene encoding a male germ-cell-specific sperm tail protein, and the organization of its genomic structure. The mRNA of the human oppo 1 gene (h-oppo 1) was expressed exclusively in the testis, and the 30 kDa protein encoded by the mRNA was detected in human testis and sperm. Immunohistochemical analyses showed that human OPPO 1 protein was localized in the flagellae of ejaculated sperm. A human genomic DNA database search indicated that the h-oppo 1 gene mapped to chromosome 17. The genomic structure of h-oppo 1 showed differences in exon/intron usage, the sequence of the 5'-flanking region, and the first intron was rich in Alu repeats as compared with the mouse oppo 1 gene. Comparison of the two genomic sequences indicated that human oppo 1 has evolved independently, resulting in substantial differences in the genomic structure after the human-mouse split, whereas the sequence of the basic functional unit of the oppo 1 gene seems to have been relatively well conserved.