Aim We tested whether the size of habitat patches along the coastline of Great Britain influences molluscan species richness.Location Coastline of Great Britain.Methods Intertidal mollusc data were compiled from the National Biodiversity Network to derive a matrix of species presence/absence in 10 km 9 10 km squares (hectads). Major groupings within the coastal fauna were identified using clustering based on Simpson's dissimilarity index. Contiguous hectads assigned to the same cluster were considered as patches. Potential island biogeographical effects were investigated using regressions of species density against patch size.Results 598 hectads were clustered into 15 groups, with the three largest groups (94% of hectads) having broad associations consistent with hectad dominance by rocky shore habitat, sheltered sediment or sediment on exposed coasts. For all three main groups, there were fewer species in larger patches than would be expected from a random sampling of hectads. Species densities (species hectad(-1)) increased with patch size in rocky shore-dominated habitat. There was no support for a similar effect in sedimentary habitats, with higher than expected species richness in isolated hectads of sheltered habitat.Main conclusions The increases in mollusc species density with patch size in rocky shore-dominated habitat are consistent with island biogeographical processes. The absence of similar effects in sedimentary habitats may reflect more overlap between the species of intertidal and subtidal in these habitats. Subtidal habitat may therefore act to change the hostility of the matrix between intertidal patches of sedimentary habitat, diluting any island effects. As landscape effects may change species richness at the scale examined, concerns that increased building of artificial habitats will change the local patterns of species richness may be justified for rocky habitats.