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Desert locusts, Schistocerca gregaria, switch between the cryptic solitarious phase and the swarming aposematic gregarious phase with increased population density. We tested whether solitarious- and gregarious-phase locusts respond differently to hyoscyamine, a plant alkaloid that protects against predators. Solitarious-phase locusts were deterred by the taste of this compound and discriminated against it in long-term choice tests. Gregarious-phase locusts readily accepted the alkaloid and fed equally from a choice of foods that did or did not contain hyoscyamine. Solitarious insects that had begun the transition to the gregarious phase (i.e. that had been crowded) accepted and even seemed to prefer food containing the alkaloid. We did not detect any physiological costs associated with consuming hyoscyamine. Solitarious-phase locusts avoid predator detection through crypsis and would not benefit greatly from consuming toxic plants. Gregarious-phase locusts follow a foraging strategy found in other aposematic grasshoppers: they switch frequently between food plants with little regard to allelochemical content, and could acquire antipredator defence by feeding on a variety of toxic plants. Solitarious locusts in transition to the gregarious phase might benefit more from chemical defence than isolated individuals because predator avoidance learning is enhanced when distasteful prey are aggregated. Phase differences in locust feeding behaviour form part of integrated cryptic and aposematic antipredator strategies that also include differences in coloration and aggregation. (C) 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.