Abstract The behaviour of Asterias rubens (L.) feeding on a selection of common infaunal bivalves was investigated on a sand bottom in aquaria. A seasonal preference for Abra alba (Wood) and Spisula subtruncata (da Costá) was noted. Nucula turgida (Leckenby & Marshall), although relatively numerous, was an insignificant component of the seastar diet. “Fitness” of prey as food species (mg AFDW of prey obtained per feeding bout) appeared to be the major factor determining preference. This was affected seasonally by accessibility of the highest ranked prey, Abra, through its increased burrowing activity. The investigation revealed that when presented with a selection of bivalves of varying accessibility, abundance, ease of opening, and food value, Asterias rubens was able to implement a flexible feeding policy consistent with the general rules of diet optimization.