In our study of the cecropin locus in Drosophila we have found a gene for a new peptide, andropin, with antibacterial properties. Transcripts from this gene, Anp, could be detected in newly eclosed males and reached steady-state levels after 1 day. Transcription was strongly induced in response to mating and is strictly confined to the ejaculatory duct of adult males. The deduced peptide sequence reveals a hydrophobic amino terminus with striking similarity to the signal peptide of the cecropins. The sequence of the predicted mature andropin shows no direct homology with the cecropins, but the two peptides may have similar secondary structures. We have synthesized the predicted gene product and shown it to be antibacterial. Crude extracts from male genital tracts show a potent bactericidal activity, and electrophoretic separation revealed at least three antibacterial components, one with the same mobility as the synthetic peptide. It appears that insects have evolved a mechanism for the protection of the seminal fluid and the male reproductive tract against microbial infections.