Despite their economic importance, only very little information is available regarding (neuro)endocrine mechanisms of reproduction in bivalve molluscs. To gain insights into the molecular control of gonadic development of these animals, G protein-coupled receptors (GPCR) expressed in the gonad of the pacific oyster Crassostrea gigas were investigated. One such receptor was cloned by RT-PCR using oligonucleotide primers derived from consensus sequences of various vertebrate (neuro)peptide receptors. This receptor named Cg-GnRH-related receptor (Cg GnRH-R) exhibits a high degree of amino acid sequence identity with both vertebrate GnRH receptors and insect AKH receptors. Quantitative RT-PCR shows a specific expression of Cg-GnRH-R in both male and female gonads during the reproductive cycle. This demonstrates for the first time the plausible involvement of a GnRH receptor orthologue in the control of reproduction in a protostomian invertebrate.