The hypothesis that risk of shell injury from crab predation and crushing by boulders and stones is important in the selection of shell thickness in the intertidal gastropod Littorina rudis Maton was tested using data from 24 populations from Britain. Although there was an obvious relationship between the incidence of shell injuries and both the distribution of crabs and substratum mobility, there was no correlation between risk of shell injury and shell thickness. This is probably accounted for by between-habitat variations in energy levels which may vary independently of crab density and substratum mobility