Pomacea (Ampullariidae) snails, commonly referred to as apple snails, serve as prey for many freshwater-dependent predators, and some species are highly invasive. Identifying limits to apple snail distribution and abundance are pertinent to understanding their ecology. Calcium (Ca 2? ) availability and pH generally influences freshwater snail populations, yet scant data exist for Pomacea snails. We measured 6-week change in shell length (DSL) in P. paludosa in two laboratory experiments with varying Ca 2? and pH levels. DSL was significantly higher in C28 mg Ca 2? /l compared with treatments B14 mg/l. Snails from populations living in low Ca 2? /pH waters did not appear genetically predisposed at growing faster in these conditions. Smallest DSL was in snails treated with 3.6 mg Ca 2? /l and pH \ 6.5 water; these snails had signs of shell erosion. Shell crush weights (CWs) were lowest for snails grown in the lowest Ca 2? /pH treatment. Smaller shells and lower CWs have implications for predation vulnerability and reproductive success. Our results are consistent with reports associating relatively low snail densities with relatively low Ca 2? /pH waters, and they are consistent with the geographic distribution of P. paludosa as related to the underlying water chemistry as influenced by geology.