This is a study of the foraging behaviour of Carcinus maenas on Mytilus edulis and of Thalamita danae on Perna viridis. Particular attention is given to differences arising in foraging behaviour as a result of intraspecific prey heterogeneity and experimental protocol. Intersite and temporal differences in the population density, shell morphology, biomass and byssal attachment strength of Mytilus edulis were found. Byssal attachment strength and shell strength were highly variable amongst individuals of a similar size. Carcinus maenas is strongly heterochelous. Intraspecific differences in the chelal mechanics, but not in the chelal geometry, were recorded; major chelae of large male crabs were significantly stronger than the major chelae of females and small males. Stomach content analyses showed that Carcinus maenas has a broad diet in which Mytilus edulis forms an important component. Intersite differences inMytilus edulis shell morphology altered the foraging behaviour of Carcinus maenas, and intersite and temporal variations in mussel flesh weight altered the prey value curves. Both C. maenas and Thalamita danae were highly prey size-selective when foraging on groups of different sized mussels, the size of prey most vulnerable to predation altering with the size composition of the group. The handling times of mussels for both species of crab were reduced when mussels were presented as part of a group as compared to when mussels were presented singly. For Carcinus maenas the reduced handling times resulted from the less extensive gleaning of mussel shells whilst for Thalamita danae reduced handling times appeared to result from the greater use of a more time efficient opening technique. When Carcinus maenas were presented with mussels of differing attachment strengths, crabs selected more weakly attached mussels over those with a more firm and rigid attachment. This selection did not appear to be based on prey value or prey length but rather on the resulting slight movement of weakly attached mussels whenever these were touched by a foraging crab.