Prey selection bySomaniathelphusa sinensis(Parathelphusidae) among five gastropod species (two prosobranchs, three pulmonates) was associated with two snail attributes. First, the degree of defence of the tissues by the shell, approximated by shell weight (SW)/tissue dry weight (M). Snails with the heaviest shells (Sinotaia quadrata: Viviparidae) could not be attacked successfully by any crabs, and those with somewhat lighter shells (Melanoides tuberculata: Thiaridae) were protected from small crabs. Secondly, among those light‐shelled snails which could be attacked successfully, prey value (M/handling time,Ht) determined rank order of preference.Biomphalaria straminea(Planorbidae), with the highestM/Ht, was the species most readily eaten. When presented with a single snail species, crabs selected among different‐sizedB. stramineain a way which maximizedM/Ht. This preference was most strongly exhibited by small crabs. A lack of size selection forRadix plicatulus(Lymnaeidae) prey was in agreement with an absence of a clear influence of size onHtfor this species. Manipulation ofM/Htby attaching plastic discs toB. stramineashells (thereby increasingHt) resulted in patterns of prey selection in accordance with the hypothesis that foragingS. sinensismaximizedM/H1.Partial consumption ofR. plicatulus, and to a lesser degreePhysella acuta(Physidae), was common when groups of snails were presented to individual crabs, and occurred more frequently when large snails were offered. Partial consumption and subsequent rejection of large snails may have masked selection for small prey which were rarely abandoned before being completely eaten.The implications of patterns of prey selection were discussed with reference to the influence of crab predation on snail distribution and abundance in Hong Kong fresh wat