Cecropins are antibacterial peptides, induced in insects in response to bacterial infections. In Drosophila, three cecropin genes have previously been characterized, CecA1, CecA2, and CecB, in a dense cluster at 99E on the third chromosome. From the same locus, we now describe a fourth member of the cecropin gene family, CecC, which is mainly expressed at the early pupal stage. In situ hybridization to immunized pupae show that CecC is induced in the anterior end of the larval hindgut and in other larval tissues that are undergoing histolysis. Within these other tissues it is often expressed in distinct foci that may correspond to hemocytes. A similar pattern of expression in the metamorphosing pupa is also observed for the CecA and CecB genes. Comparing the DNA sequences of the cecropin genes, a conserved region is observed about 30 bp upstream of the TATA box. It consists of three shorter motifs, two of which are reminiscent of a putative promoter element in immune protein genes from the cecropia moth.