The Rel/NF-kappa B transcription factor Relish performs a central role in the acute-phase response to microbial challenge by activating immune antibacterial peptides. We cloned and molecularly characterized the gene homologous to Drosophila Relish from the mosquito Aedes aegypti. Unlike Drosophila Relish, Aedes Relish has three alternatively spliced transcripts encoding different proteins. First, the predominant Aedes Relish transcript of 3.9 kb contains both the Rel-homology domains and the inhibitor kappa B (I kappa B)-like domain, which is similar to Drosophila Relish and to the mammalian p105 and p100 Rel/NF-kappa B transcription factors. Second, Aedes Relish transcript contains Rel-homology domains identical to those of the major transcript but it completely lacks the I kappa B-like domain-coding region, which has been replaced by a unique 3'-untranslated region sequence. In the third transcript, a deletion replaces most of the N-terminal sequence and Rel-homology domains; however, the I kappa B-like domain is intact. All three Aedes Relish transcripts were induced by bacterial injection but not by blood feeding. In vitro-translated protein from the Rel-only construct specifically binds to the kappa B motif from Drosophila cecropin A1 and Aedes defensin genes. PCR and Southern blot hybridization analyses show that these three transcripts originated from the same large inducible mRNA encoded by a single Relish gene.