In this study, we have isolated and fully sequenced a homologue of the vertebrate Brn-3 gene from the bivalve mollusc Pecten maximus (L.), which we have called PmaPOU-IV. The derived amino acid sequence exhibits characteristic features of POU-IV proteins. First, the new sequence includes a POU-specific domain and a POU-homeodomain. Second, PmaPOU-IV contains a POU-IV box, found only in the proteins of class IV. Third, the Pecten POU-IV bipartite domain shares a significant amino acid sequence identity with other members of the POU-IV class. Finally, phylogenetic analyses confirm that PmaPOU-IV is an orthologue of POU-IV genes. We have also investigated by reverse transcription quantitative real time PCR (RT-qPCR) the expression pattern of PmaPOU-IV in adult tissues. PmaPOU-IV was expressed in several sensory organs and gonads but was undetectable in both muscle and the digestive gland. Our findings support the belief that POU class IV genes involved in the terminal neuronal differentiation are still operating in the adult nervous system.