Insects express a battery of potent antimicrobial proteins in response to injury and infection. Recent work from several laboratories has demonstrated that this response is neither stereotypic nor completely nonspecific, and that different pathways are responsible for inducing the expression of antifungal and antibacterial peptides. Here we report the cloning of two closely linked attacin genes from Drosophila melanogaster. We compare their protein coding sequences and find the amino acid sequences to be more highly conserved than the nucleotide sequences, suggesting that both genes are expressed. Like other antimicrobial peptides, attacin expression is strongly induced in infected and injured flies. Unlike others, attacin transcription is uniquely sensitive to mutations in the 18-Wheeler receptor protein, and thus may be regulated by a distinct signaling pathway. The number and organization of binding sites for kappaB and other transcription factors in the promoter regions of both attacin genes are consistent with strong and rapid immune induction. We demonstrate that these promoter regions are sufficient to direct beta-galactosidase expression in transformed Drosophila third-instar larval fat body in a bacterially inducible manner. We present a comparison of the promoter regions of the two attacin genes to those cloned from other antimicrobial peptide genes to assist a better understanding of how antimicrobial genes are differentially regulated.