In summer, oysters suffer mortality that decimates this year about 60% of the population in France. We previously demonstrated a genetic basis for summer survival thatallowed us to produce resistant (R) and susceptible (S) oyster familiesby divergent selection (Samainet al., 2007). At the physiological level, R families can survive summer mortality because they are reproductively less active than S families (Fleury et al., 2008).In order to identify the molecular basis of the summer survival,a transcriptomicapproach on R and S families was performed using microarray analysis and 34 differentially-expressed genes were identified between R and S in the gonad (Fleury et al., in prep). Among these genes, we found an EST encoding a neuropeptide Y related receptor (Cg-NPYr). Due to NPY major role in the coordination of energy balance and reproduction in other species,we studied Cg-NPYrexpression and regulation in the gonad to gain insight into itsfunction and its putative role in summer mortality.