Sea bass (Dicentrarchus labrax) fisheries are becoming increasingly abundant along eastern Atlantic coastlines, especially in the ICES sub-areas IV, VII and VIII, but they are still not being managed by the European Union TAC (Total Allowed Catches) system. Scientific information about these sea bass stocks is limited, but the fishery is currently managed or modelled based upon these data, separated into the ICES divisions. Clearly, failure to manage satisfactorily the fishery may lead to a decline in stocks. We therefore investigated the structure of Atlantic sea bass populations at eight microsatellite loci in the Bay of Biscay and the English Channel and, additionally, at five selected loci in Ireland and Scotland. Genetic data showed no significant population differentiation between the Bay of Biscay and the English Channel samples, either for juvenile or adult individuals, indicating substantial gene flow. These results contrast with tag-recapture data that indicated restricted movements of individuals within the Bay of Biscay or the English Channel, and little exchange between them. These apparently contradictory genetic and tag-recapture results can be reconciled by various aspects of sea bass biology, factors that should be considered in stock management. Furthermore, the results indicate some local genetic differentiation in the Irish sample when compared to other samples, possibly indicating a complex population structure of sea bass around the British Isles. This point should be further investigated, but results clearly indicate that sea bass stock management should not be based only upon the currently recognized ICES divisions. (c) 2006 Elsevier B.V. All rights reserved.