A potent inducible antibacterial peptide carrying an O‐glycosylated substitution has recently been isolated from Drosophila[Bulet, P., Dimarcq, J. L., Hetru, C., Lagueux, M., Charlet, M., Hegy, G., Van Dorsselaer, A. and Hoffmann, J. A. (1993)J. Biol. Chem. 268, 14893–14897]. Here we report cloning studies that show that Drosophila contains a single, intronless gene, located at position 51C1–6, which encodes the precursor protein from which drosocin is processed. The upstream and the downstream sequences of the drosocin gene contain putative cis‐regulatory elements similar to mammalian regulatory motifs, namely three κB‐related decameric sequences. The drosocin gene is silent in naive animals, and is strongly induced with acute phase kinetics after immune challenge in larvae and in adults. We have established several transgenic fly lines in which reporter genes were placed under the control of various drosocin promoter sequences. Our results indicate that 2.5 kb of upstream sequences confer inducibility and tissue specificity to the transgene, but that the level of its expression in the fat body after immune challenge is low. Addition of genomic regions downstream of the drosocin transcribed sequences results in increased transcription levels, which are similar for the fusion and the resident drosocin genes upon infection. Analysis of transgenic fly lines showed that the drosocin reporter gene is constitutively expressed in the oviducts of egg‐laying females.