Snails in the family Pleuroceridae have strong effects on community structure and energy and nutrient pathways in many North American rivers and streams. Identifying factors determining densities and biomass of pleurocerids is, therefore, an important step toward predicting the relative importance of these snails in an ecosystem. We used plot sampling to identify environmental factors causing variation in demographic features of the pleurocerid snail Leptoxis carinata in a 3rd order stream in south central Virginia. Snails were collected from forty 0.22 m² plots located within a 165 m segment of Fishpond Creek and L. carinata density, mean shell width, mean body mass and total biomass in each plot were measured. Measurements were also taken for several environmental variables. Multiple regression indicated that topographic complexity of substratum was the primary cause of variation in L. carinata density and total biomass among plots. Snail density and total biomass were both greater in topographically complex plots with large substratum particles (i.e., boulders, cobble) that likely provide refuge from high flow events and are preferred habitat for foraging. Temporal changes in several demographic features were also quantified. Mean shell width and body mass of individual snails increased during the 28 d study (12 March 2001–8 April 2001), but total biomass declined following a flood that occurred from 25 March 2001–5 April 2001.