Understanding the extent to which traits that are used to delimit and diagnose species are phenotypically plastic is important for recognizing species boundaries. Shell characters have long been used for describing species of gastropods, even though these features may be influenced by environmental conditions. To determine the degree of phenotypic plasticity of two North American lymnaeid species, Stagnicola elodes (Say, 1821) and Stagnicola emarginata (Say, 1821), that occur in different habitats and differ in shell morphology, we reared individuals in captivity under similar conditions and compared shell shapes and sizes of wild-caught and captive-reared populations. We also exposed individuals of S. elodes to effluent from a potential predator (crayfish) to gauge the possible impact of the presence of predators on shell morphology. Although the two species remained morphologically distinct, shell shapes of captive-reared individuals of both species differ significantly from those of wild-caught individuals and show similar magnitudes of change among species. Directions of change, however, differed significantly among species. Although shell shapes of individuals of S. elodes that were exposed to crayfish cues were not significantly different from control snails, shell sizes of exposed snails were smaller than unexposed snails. These results suggest that exposure to predators affects growth rates of S. elodes. Nonetheless, given significant associations between shell shape and size that were observed in the captive-rearing and predator-exposure experiments, shell shape changes allometrically during development. These results suggest that morphological differences of other North American Stagnicola species reflect ecophenotypic variation, but more work is necessary to further evaluate this hypothesis.