Cecropins A, B, and D are antibacterial peptides of 35-37 amino acids that are synthesized in pupae of the Cecropia moth (Hyalophora cecropia) as a response to a bacterial infection. cDNA cloning has shown that the cecropins are made as preproproteins that are processed in four steps to the mature peptides. We have now cloned the genes for preprocecropins A and D, data that together with earlier work on the B gene has made it possible to deduce the arrangement of the cecropin locus. The genes for the three cecropins are organized in a large cluster spanning 20 kilobases of DNA and for each gene there is one copy/haploid genome. The size of the cluster is in part due to long distances between the genes and to the presence of insertion elements in the introns of the A and D genes. The cecropin genes are not expressed in parallel. Transcripts for cecropins A and B appear within 2 h after injection of live bacteria, they reach a maximum after 48 h, and they are continuously expressed at this level for several days. The D gene has a delayed pattern of expression where transcripts appear within 48-96 h and reach a maximum after 144 h. In consonance is also the production of the mature cecropins A, B, and D where the active cecropins A and B are detected in the hemolymph within 10-24 h while the D form is not detected until 48 h post infection. Control injections with sterile saline produced only a weak induction of the cecropin genes.