Marine invertebrates are exploited increasingly as food resources worldwide, but knowledge of genetic population structure is limited for most commercially valuable taxa. We investigated genetic structure in the widely distributed, subtidal gastropod Buccinum undatum by screening samples from 28 locations at 5 microsatellite loci. Multidimensional scaling and cluster analysis were applied to study macrogeographic structure, and revealed 4 well-supported groupings of populations: Canada, Iceland, Swedish Skaggerak, and a cluster containing most samples from the European continental shelf (hereafter 'Shelf'). A fifth cluster comprising all 5 samples from the Solent (UK) relied on exceptional differentiation of just 2 alleles, which is consistent with significant evidence for recent bottlenecks in this area. Within the Shelf cluster most pairwise tests of differentiation were significant, although the global F-ST of 0.014 was very low for a direct-developing species. Our data suggest that use of highly polymorphic markers caused a relatively minor downward bias to F-ST, although historical connectivity of populations that are not in migration-drift equilibrium might be more important. However, significant isolation by distance among British North Sea coast samples (FST = 0.010) is consistent with approach to equilibrium and suggests recent gene flow, probably between semi-continuous populations. At a microgeographic scale, we found migration to be consistently higher from inshore to offshore within 3 separate areas; a factor that may underpin the lower diversity and greater differentiation observed for bay and inlet populations. Such populations might serve as important sources of genetic diversity, but are likely to be particularly vulnerable to exploitation.