The Marine Nature Reserve of Lough Hyne (south-west Ireland) encompasses an extremely wide range of marine conditions within a small area. A total of 378 boulders were collected from a range of depths at five sites subjected to very different degrees of water movement and sedimentation. Encrusting biota were identified and the communities compared between sites and upper and lower surfaces. In all, 114 taxa were identified. Alpha diversity was generally found to be higher for lower surfaces at sites with intermediate flow/disturbance. Sites with high water flow had much greater space occupation, predominantly cheilostomatid bryozoans. Multivariate analyses divided the samples into six groupings. Similarity percentage analysis (SIMPER) was used to indicate particular species characteristic of each group. Equations for the species-area curves generated tended to indicate communities at equilibrium except at sites subjected to stresses such as high sediment load or high disturbance. For high disturbance sites it was apparent that number of species cm−2 was significantly negatively correlated with surface area of rock implying non-random distribution.