The common European freshwater snail Lymnaea peregra is a morphologically very variable species. A field survey in natural ponds showed that snails had a rounder shell shape in ponds with fish than in fishless ponds. A subsequent study of the reaction norm in this snail was made in a common garden experiment with snails from several populations and ponds with and without fish. Their offspring was raised either in presence or absence of fish chemical cue. The results showed parallel reaction norms in shape of snails irrespective if they originated from fish ponds or fishless ponds. Thus, differences in shell shape is a phenotypically plastic response to local predation regimes. The plastic response also incurs fitness constraints. In a density gradient of snails, decreased growth rate and a severe reduction of egg number was shown in snails exposed to fish chemical cues compared to controls.