IN trying to understand the structure of ecological communities, ecologists usually pay particular attention to the interactions between pairs, or small groups of species1. Questions about the 'shape' of the food webs within which these species are embedded are much more rarely asked 2-4. For example, what happens when a population feeds at more than one trophic level (omnivory)? In some real food webs there seem to be no omnivores (Fig. 1a)5; in others omnivores are common6,7 (Fig. 1c)8. In this note we attack the problem of omnivory using simple, linear Lotka-Volterra models of food webs9, and show that certain patterns are much more likely to persist on an evolutionary time scale than others. We then compare the model predictions with real food webs. © 1978 Nature Publishing Group.