An enclosure-exclosure experiment was performed in Nonquit Pond, Rhode Island to test the effect of grazing byAmnicola limosa(Say) (Gastropoda) upon lentic periphyton. Periphyton from enclosures withA. limosahad higher organic content in May and July, and lower standing crops in May, July, August, and September than periphyton from exclosures withoutA. limosa.Small diatom (< 18 μm long, other thanCocconeis placentula) abundances on glass slides were significantly lower in the enclosures than in the exclosures.Cocconeisabundances were not significantly affected by grazing.Cocconeisrelative abundances on glass slides exposed in the littoral zone of Nonquit Pond also increased with increased grazing pressure fromA. limosa.Because characteristics of Nonquit Pond do not seem to be unusual, and grazer abundances were not extreme, these data may have general importance to the study of lentic periphyton community structure.