Chemoreception in marine invertebrates mediates a variety of ecologically important behaviors including defense, reproduction, larval settlement and recruitment, and feeding. The sensory pathways that regulate deposit-feeding activity by polychaetes living in sedimentary habitats are of particular interest because such feeding has profound effects on the physical and chemical properties of the habitat. Nevertheless, little is known concerning the molecular mechanisms of chemical signal transduction associated with deposit feeding and other behaviors in polychaetes. Chemosensory-based feeding behaviors are typically regulated by G-protein-coupled signal transduction pathways. However, the presence and role of such pathways have not been demonstrated in marine polychaetes. Methodologies involving degenerate primer-based reverse transcription with the polymerase chain reaction and rapid amplification of cDNA ends were used to identify and characterize a Galphaq subunit expressed in the feeding palps of the spionid polychaete Dipolydora quadrilobata. The D. quadrilobata Galphaq protein had high sequence similarity with previously reported Galphaq subunits from both invertebrate and vertebrate taxa. Immunhistochemistry and immunocytochemistry were used with confocal laser scanning microscopy and transmission electron microscopy to visualize the distribution of a Galphaq antibody in whole worms and in cilia of the feeding palps. Galphaq immunoreactivity was concentrated in the nuchal organs, food-groove cilia, and lateral/abfrontal cilia of the feeding palps. Because these structures are known to be involved in chemoreception, we propose that Galphaq isolated from D. quadrilobata is a key component of chemosensory signal transduction pathways in this species.