The development of attached algal communities in a lowland stream in central Sweden was investigated with reference to differing levels of grazing pressure. An enclosure-exclosure method was used to measure periphytic biomass accrual and community structure in the presence of low, natural, and high densities of the gastropodTheodoxus fluviatilis. A highly significant difference (p < .01) in chlorophylla, carbon, and nitrogen was observed between chambers with low grazer density and those with natural and high grazer densities. Chambers with natural and high densities ofT. fluviatilishad lower percent composition of filamentous green algae (mainlyCladophora) and lower C/N ratios in the periphyton.