Spatial distribution patterns of two sessile gastropods belonging to Vermetidae, Vermetus triquetrus and Dendropoma petraeum, were studied in June 2001 in north-eastern Sardinia (Mediterranean Sea, Italy). Density of individuals was assessed within quadrats positioned on a shallow (0·1–1 m depth) rocky substrate of different lithological types (limestone and granite), at a hierarchy of spatial scales. Density of D. petraeum is significantly greater on granite than on limestone, while V. triquetrus does not show any differences related to rock type.