Food intake, feeding efficiency, prey profitability and diet choice were examined in wild-caught three-spined sticklebacks, Gasterosteus aculeatus, infected with the cestode Schistocephalus solidus. Infected fish ingested significantly fewer chironomid larvae (bloodworms) before becoming satiated and handled prey for significantly longer than did uninfected fish. This was especially marked when fish had partially-full stomachs. For uninfected fish, large bloodworms were more profitable than smaller ones, regardless of level of satiation. The two prey sizes were equally profitable for infected fish with empty stomachs. However, for infected fish that had already consumed one prey item, smaller bloodworms were more profitable; prey profitability was negatively related to relative size of the parasite. When offered a choice between large and small bloodworms, uninfected fish consistently chose the larger. Infected fish with empty stomachs were unselective with respect to prey size, but after eating just one prey item, they showed a significant preference for the smaller prey.