Optimal foraging theory was used to explain selective foraging by the introduced signal crayfish (Pacifastacus leniusculus) on the thin-shelled common pond snail (Lymnaea stagnalis). Crayfish predation efficiency was studied in relation to habitat complexity and snail size. In a pool experiment (area 1.3 m2) single adult crayfish were allowed to feed on four size classes of snails for one week. A pair-wise preference trial (aquarium experiment) tested if adult crayfish selectively predated on particular size classes of snail and if prey value (expressed as snail dry mass per handling time) could explain the size range of snails chosen. Crayfish preferred the smallest size classes of snails in both pool and aquaria experiments. In the pool experiment crayfish had a strong effect on snail survival. Habitat complexity did not affect overall snail survival, but resulted in reduced predation pressure on the smallest size classes of snails. Handling time and shell-thickness increased exponentially with increasing snail size, and the two smallest size classes had the highest prey values. The results suggest that crayfish can structure the abundance and size distribution of thin-shelled snails, through size-selective predation and reduction of macrophytes. The mechanisms behind the choice of snails may be based on prey value and reduced exposure time to predators and conspecifics. Crayfish effects on snail size distribution may be less pronounced in complex habitats such as macrophyte beds.