ABSTRACT: Randomly amplified polymorphic DNA (RAPD) banding patterns were compared between samples of the same year class of the scallop Pecten maximus (L.) from 5 locations (beds) within the commercial fishing grounds around the Isle of Man (UK). Phenotypicanalysis of molecular variance (AMOVA) indicated that significant differentiation was present between these beds, although this accounted for only 2% of total variation, the remainder being between individuals within beds. As confirmed by multivariateanalyses (PCOORD and UPGMA clustering), samples from the 2 northern Isle of Man beds resembled each other particularly closely, as did 2 southwestern beds, while the fifth, East Douglas, was relatively distinct. Comparison was extended to 2 samples takenoutside the Irish Sea, from Mulroy Bay (Co. Donegal, Eire) and Plymouth (southwest England). Differentiation between the 3 regions was significant, accounting for 7% of total variation in a data set with balanced regional representation. A Mantel test onthe whole data set revealed no significant correlation of phenotypic distance, based on RAPD banding pattern, with geographic distance. The potential correlation was largely destroyed by the marked differentiation of the population in Mulroy Bay, asemi-enclosed sea lough, and by the unexpectedly high phenotypic similarity between the Plymouth sample and the 2 northernmost Isle of Man samples. The RAPD data presented here provide the first evidence of population genetic structuring in exploitedopen-water stocks of this species, since previous allozyme studies of P. maximus have indicated genetic uniformity. Differentiation of the Mulroy Bay population from open-water stocks has been demonstrated previously in a study of mtDNApolymorphisms.