Comparative studies were conducted on intertidal algal and faunal communities associated with Sargassum muticum and native algae on the Isle of Cumbrae, Scotland. Significant differences were detected in the structure and composition of the communities. The abundance of the dominant native algae Dictyota dichotoma was found to be reduced in areas dominated by S. muticum and displacement through competition for substrate or light is inferred. Elevated faunal abundances were associated with S. muticum and evidence indicates that increased detrital input may be a possible contributory factor. Differences in faunal community composition were most pronounced in the sessile epifauna below the S. muticum canopy. Faunal communities associated with S. muticum were less heterogeneous in composition than those associated with native algae at equivalent shore positions. Some of the compositional differences may be coincidental and attributable to factors favouring the establishment of S. muticum rather than to the direct influence of the S. muticum canopy.