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The behaviour of herbivorous insects is influenced by their nutritional state. Nutrition-induced behavioural changes are often interpreted as adaptive mechanisms for controlling nutrient intake; however, their influence on other life history traits has received far less attention. We investigated the effect of food quality and distribution on the behaviour and phase state of desert locusts, Schistocerca gregaria Forskal (Orthoptera, Acrididae), which change from the ’solitarious’ to the ’gregarious’ phase in response to population density. Phase change involves many morphological, physiological and behavioural changes. Solitarious insects are cryptic whereas gregarious locusts aggregate. Individual phase change is stimulated by mechanical contact with other locusts. A clumped resource distribution promotes change to the gregarious phase by increasing crowding and contact between individuals. In this study, we found that the effect of food distribution on locust phase depended on the nutritional quality of the food. We used three synthetic food treatments: near optimal, dilute and a choice of two unbalanced but complementary foods. Clumped resource distribution led to increased gregarization in the dilute and the complementary diet treatments. This effect was particularly pronounced on the complementary foods, owing to the interaction of crowding and locomotion. Gregarization was most pronounced in the dilute diet treatment, owing to increased activity. These diet-induced effects are explained in terms of behavioural changes in locomotion, quiescence and feeding that are consistent with what is known from earlier work on locust feeding behaviour and behavioural phase change. (C) 2000 The Association for the Study of Animal Behaviour.