Andropin, which encodes an antibacterial protein, is closely linked to the Cecropin gene cluster of D. melanogaster. Andropin and Cecropins are considered to have originated from one common ancestor. However, the expression pattern of Andropin is distinct from that of Cecropins, being restricted to the adult male ejaculatory duct. To elucidate the evolutionary process of Andropin, we have sequenced Andropin genes from D. melanogaster and its closely related species. In D. melanogaster, the nucleotide diversity of Andropin is remarkably low compared to that of Cecropin. In contrast, nonsynonymous substitutions of Andropin are conspicuously frequent between species. From genomic Southern analysis, Andropin-like genes are present in at least the melanogaster species subgroup. The series of present results suggests that Andropin was born in the course of constructing the Drosophila Cecropin gene family and then started to evolve rapidly, in contrast to Cecropins.