Almost all knowledge of the regulation of seg-mentation genes in arthropods comes from Drosophila. In order to study the regulation of the segment-polarity gene engrailed in a non-insect arthropod we focussed on puta-tive regulatory regions of the engrailed.a (en.a) gene in the barnacle crustacean Sacculina carcini. In this animal, en.a is expressed in segmental stripes like the engrailed genes of other arthropods. As transgenesis in Sacculina is not possible at present, we have used Drosophila as a test tube. The Sacculina en.a intron is able to induce a specific expression of lacZ in the Drosophila wing imagi-nal disc. This pattern is not an engrailed-like pattern, but does suggest that some Drosophila transcription factors interact with the Sacculina en.a intron. We show that two Drosophila POU proteins, Nubbin and VVL, and Eng-railed itself bind to the Sacculina en.a intron in vitro and that they regulate this expression in vivo. The conservation of POU protein binding sites in metazoans suggests that Sacculina POU proteins could recognize the same sequences. Hence, we looked at the expression of nubbin and vvl homologues in Sacculina larvae. Indeed, their expression patterns are consistent with a putative regulatory function on en.a in segments and appendages. Remarkably , the vvl homologue is expressed in Sacculina in a striking striped pattern that is very different from the vvl pattern in Drosophila embryos, and is complementary to the Sacculina en.a pattern. These experiments suggest that the Sacculina engrailed.a gene is regulated by POU proteins.