We collected aquatic gastropods at 137 sites in lakes and streams of Indiana and tested for patterns of assemblages with environmental variables. The survey resulted in 32 species with a mean of 2.8 species at each site, and a mean abundance at each site of 144 individuals. Nonmetric multidimensional scaling (NMS) multivariate analyses resulted in watershed drainage area, water conductivity, substrate category frequency, and dissolved oxygen as significant correlates of gastropod assemblage structure. Gastropod assemblages of lakes were not significantly different than assemblages of streams in the ordination. Prosobranch taxa occurred in higher abundances than pulmonate taxa at sites with lower conductivity in larger watersheds. There were no pairs of gastropod species that tended to co-occur more frequently than random. Our analyses resulted in local environmental variables providing explanation of aquatic gastropod assemblage structure.