Micro-scale (≤1 m) and meso-scale (1 m-50 m) spatial heterogeneity in infaunal benthic communities and their sedimentary environment were measured at two sites on an intertidal muddy-sand shore in the Clyde Estuary, Scotland. The low tide site was a high energy erosional environment dominated by large sand waves. The high tide site was a low energy depositional environment dominated by patchy algal mats of Enteromorpha. Two sediment indicator parameters (shear strength, redox potential) and the abundance of eight infaunal indicator species (Arenicola marina, Bathyporeia guilliamsoniana, Corophium volutator, Fabricia sabella, Hydrobia neglects, Macoma balthica, Nereis diversicolor, Pygospio elegans), were measured. Meso-scale spatial variability was higher than micro-scale variability at both sites, and peaked at c. 10 m at low tide. This peak is probably related to the scale of the sand waves at low tide. In general, variability in sediment parameters and in species abundance was higher at high tide than at low tide. There were fewer significant correlations between variables at high tide (17/78 = 22%) than at low tide (20/45 = 44%). Community diversity (Shannon-Wiener, Simpson) was higher but varied less at high tide; this may be causally related to the greater variability of, and lack of correlations between, the variables at high tide, and to the presence of algal mats which had marked effects on the abundance of some species. In general, it may mean that in low energy environments, increased spatial variability in and fewer correlations between species abundance and sediment parameters produce a more diverse but less variable community structure. These results are highly significant to our understanding of spatial heterogeneity in infaunal macrobenthic communities on shores, and give a deeper insight into the factors governing community diversity in different sedimentary environments.