In recent years it has become clear that intraguild predation (where predators feed on other predators) has important consequences for food webs, and yet very little is known about its nutritional or functional bases. In the most detailed study of the nutritional basis of foraging by a predator to date, we used geometrical analysis to test the ability of the generalist invertebrate predatory beetle, Agonum dorsale (Carabidae), to forage selectively for lipid and protein over a 10-day period following emergence from winter diapause, and we measured associated changes in body lipid and nitrogen content. Over the first 48 hours, beetles that were offered two nutritionally imbalanced but complementary foods self-selected a diet high in lipids, and thereafter the proportion of protein in the selected diet increased. Beetles confined to a single food with excess lipid (higher lipid:protein ratio than the self-selected diet) regulated intake to meet lipid requirements, while suffering a shortfall of protein. Those given diets with a lower lipid:protein ratio than the self-selected diet showed a progressive tendency across the 10-day experiment to over-ingest protein, thereby reducing the lipid deficit in their diet. Body composition changed markedly during the experiment, with the lipid content of the self-selecting insects increasing over the first 48 hours from 14% to 46% by dry mass, and thereafter remaining stable. We discuss some implications of our results for the understanding of intraguild predation.