Four categories of plant food dominated the diet of chimpanzees in Kibale Forest, Uganda: non-fig tree fruits, fig tree fruits, herbaceous piths and terrestrial leaves. Fruit abundance varied unpredictably, more among non-figs than figs. Pith intake was correlated negatively with fruit abundance and positively with rainfall, whereas leaf intake was not influenced by fruit abundance. Piths typically have low sugar and protein levels. Compared with fruits and leaves they are consistently high in hemicellulose and cellulose, which are insoluble fibres partly digestible by chimpanzees. Herbaceous piths appear to be a vital resource for African forest apes, offering an alternative energy supply when fruits are scarce.