The invasive New Zealand mud snail, Potamopyrgus antipodarum (Gray 1843), has been present in the Laurentian Great Lakes for at least 20 years. Recently, the snail has spread to streams that empty into Lake Ontario. In the deep waters of Lake Ontario, detritus is the common food source for the snails; while in streams periphyton becomes an important part of their diet. This experiment was designed to test the hypothesis that a diet of periphyton will lead to a higher growth rate compared to detritus. Benthic sediment was collected from 18 m depth in Lake Erie. Periphyton was grown on small rocks collected from Spring Run stream in central Pennsylvania. Eighty juvenile snails (0.6–1.0 mm in length) were placed individually into plastic cups and divided into four treatments: control snails that were fed Spirulina powder (standard lab diet), snails kept in lake sediment, snails reared on rocks with periphyton growth, and snails in a combination habitat of rock and lake sediment. After six weeks, the snails were measured again. Snails in the periphyton-only treatment had a significantly greater growth rate than snails in the remaining treatments. The other three treatments resulted in similarly low growth rates. These results suggest that the change in diet from detritus-based to periphyton-based as the snails move from a lake to a stream environment may result in higher individual and possibly population growth rates, and thus could increase their invasion success.