We are using the PCR-based differential display technique to isolate genes which are induced during the immune response in Drosophila. In this way, a cDNA clone for a member of the attacin family of antibacterial proteins was isolated. The corresponding Attacin A (Att A) gene is localized at 51A-B on the second chromosome, and it is closely linked to at least one more cross-hybridizing gene. Injection of bacteria induces a 0.8 kb transcript, with expression kinetics similar to that of cecropin. Drosophila attacin is most closely related to sarcotoxin II of Sarcophaga peregrina, but it lacks the extra domains that are unique to this protein, and the overall domain structure of the Att A gene product is identical to that of the attacins from Hyalophora cecropia.