Among arthropods, Cirripedia (barnacles) are remarkable in that they completely lack abdominal segments. This feature prompted us to study theHoxgenes of three cirripede species, representing a wide array of the diversity of these organisms, a segmented sessile barnacle,Elminius modestus(Thoracica), the parasite of a crab,Sacculina carcini(Rhizocephala), and the burrowing barnacleTrypetesa lampas(Acrothoracica). Using PCR amplification of genomic DNA and cDNA and library probing, we have found seven clear cirripedian homologues of the eight homeoticHoxgenes known in insects, includinglabialandproboscipediahomologues, that were not previously reported in crustaceans. In addition we have isolated a divergentAntp-like gene, namedDiva, that we homologize to theftzgene of insects. The homeotic geneabdominalA(abdA) was not retrieved from any of these three cirripede species. By contrast, we have found all eight homeotic homologue genes, includingabdA, inUlophysema oeresundense, a crustacean possessing a well-developed abdomen, belonging to the Ascothoracica, generally thought to be the sister group of Cirripedia. Since we have found in barnacles homeobox-containing genes that are more divergent from theAntennapediatype than the typicalabdA, we believe that abona fide abdAgene would not have escaped our search. Hence, theabdAgene has been lost or is profoundly derived in sequence during the evolution leading to the cirripedian lineage. If confirmed, the lack ofabdAwould represent the first case in which the loss of a homeotic gene is correlated with a change in body plan during the evolution of metazoans.