Macrofaunal community structure was examined in a shallow sublittoral sea grass bed (Zostera marina L.) and adjacent unvegetated sediment in the River Yealm, Devon, south-west Britain, on a single sampling occasion in August 1995. The presence of Zostera had a significant influence on macrobenthic community structure in the River Yealm. Samples from inside the sea grass bed were characterized by significantly higher numbers of individuals and species, as well as by greater species richness and high faunal dominance (i.e. low evenness) compared with the unvegetated samples. High faunal dominance within the sea grass bed was attributable to the large numbers of a few species (e.g. the polychaetes juvenile capitellids, Spio filicornis, Exogone hebes, Nereis pelagica and the tanaidacean Apseudes talpa), while increased species richness was a result of a large number of additional rare (1–2 individuals) species. The greatest number of individuals was found in samples with the highest sea grass biomass. Multivariate ordination revealed a distinct separation of macrofauna communities from vegetated and unvegetated sites. Significance testing with ANOSIM shows that there were significant differences in community structure between vegetated and unvegetated habitats.