Drosophila shows an immune response when challenged by injection of low doses of bacteria. To date, the molecules involved in this immune reaction have remained elusive, with the exception of cecropins (4-kDa antibacterial peptides initially isolated from the moth Hyalophora cecropia) for which three closely related genes have been characterized recently. We report the molecular cloning and sequencing of a cDNA from a library of immune Drosophila which encodes a novel member of the family of diptericins (9-kDa antibacterial peptides initially isolated from the fly Phormia terranovae). Transcripts for the Drosophila diptericin are detected 2 h after injection of bacteria. They are apparently derived from a single gene mapping at position 56 A on the right arm of the second chromosome. We discuss the existence of a distant relationship between the diptericins and two other groups of anti-bacterial insect proteins, the attacins, and the sarcotoxins II.