The benthic communities on individual rocks (large pebble to large cobble) were sampled in a third-order stream to examine the relationship between macroinvertebrate distribution and surface area. The number of individuals, number of taxa, and species diversity were correlated significantly with rock surface area. The number of taxa per 100 cm2was significantly greater on the smaller substrates. Abundance of each of the seven dominant taxa was also correlated with rock size, but only two taxa (Pseudocloeonsp. andEpeorussp.) showed distinct preferences for larger substrates. Multivariate analysis of these data, based on abundance of dominant taxa, indicated high similarity in distribution of macroinvertebrates on intermediate cobble substrate.