The mammalian transcription factor NF-kappa B regulates a number of genes involved in immune and acute phase responses, by interacting with a nucleotide sequence element, the kappa B-motif. In this work we demonstrate the participation of similar motifs in the immune response of insects as well: kappa B-like motifs have a regulatory role in the synthesis of cecropins, a set of anti-bacterial peptides, triggered by the presence of bacterial cell wall components in the insect blood. We show that the upstream region of the Cecropin gene CecA1 contains elements responsible for inducible and tissue-specific expression. Furthermore, a trimer of kappa B-like motif confers high levels of inducible expression from the reporter gene, after transfection in a Drosophila blood cell line. As in the moth Hyalophora cecropia, stimulation with bacterial lipopolysaccharide induces a nuclear factor that specifically binds to the kappa B-like motif. Our data suggest a functional and evolutionary relationship between these insect immune response factors and the mammalian NF-kappa B.