Experiments were conducted in a small northern Michigan lake to assess the effects of grazing by the snail Physa integra on periphyton communities. Periphyton was allowed to colonize glass microscope slides in enclosures for 26 d prior to the introduction of snails at three densities. Snails were allowed to graze for 21 d before the periphyton was collected and analyzed. Physa integra significantly reduced the biovolume, species richness, and diversity of periphyton. Diatoms were relatively more abundant and green algae relatively less abundant at low grazer densities when compared with controls and periphyton at high grazer densities. Grazers reduced the physiognomic diversity of periphyton which led to a community dominated by prostrate diatoms. Seventeen algal taxa were significantly reduced in relative abundance by grazers. Synedra and Mougeotia were the most abundant taxa of this group. Eunotia pectinalis var. minor, a prostrate chain-forming diatom, was least affected by grazing and experienced a significant increase in relative abundance in communities exposed to grazers. Oedogonium had a significantly higher relative abundance under high grazer dnesities than under either the control or low grazer density conditions. We conclude that Physa integra reduces algal densities, species richness, species diversity, and physiognomic complexity and leads to a community dominated by grazer-resistant taxa (Eunotia pectinalis var. minor and Oedogonium sp.).