Taste sensation and food selection by animals can change adaptively in response to experience, for example to redress specific nutrient deficiencies1. We show here, in two species of caterpillar, that infection by lethal parasites alters the taste of specific phytochemicals for the larvae. Given that these compounds are toxic to the parasites and are found in plants eaten by the caterpillars, their changed taste may encourage parasitized caterpillars to increase consumption of plants that provide a biochemical defence against the invaders.