After demonstrating that rats were capable of discriminating between the same diet treated with either flavour 1 or flavour 2 and that the 2 flavours of diet were equipreferred, an experiment was carried out to see whether learned taste aversion might play a role in the reduction of food intake that is commonly observed during the course of a parasitic infection. The results showed that rats, given a subcutaneous inoculation of approximately 6000 third-stage larvae of Nippostrongylus brasiliensis (Nematoda) while feeding on a diet containing flavour 2, strongly preferred to eat diet containing flavour 1 when given simultaneous choice conditions. Uninfected rats showed no preference and ate equal amounts of both flavoured diets. This effect is interpreted as the first experimental demonstration of learned taste aversion using a eukaryotic parasite as the inductive agent.