Dietary choice of some vertebrate and invertebrate herbivores in mediterranean-type ecosystems in South Africa, France and California was examined in relation to the nutritional chemistry of their food plants. The general hypothesis of this dissertation was that very low soil nutrient levels in mediterranean ecosystems results in nutritional imbalances in the food plants of herbivores both from shortages of beneficial components and from increases in deleterious secondary metabolites. Emphasis was placed on assessing the nutritional quality of food plants and evaluating feeding patterns from evidence on individual plants in the vegetation. The major findings are as follows: High concentrations of leaf tannin polyphenols appear to be a characteristic of these three mediterranean shrublands. Antelope feeding preferences in the South African shrubland were significantly and negatively correlated with leaf tannin polyphenols rather than positively with any nutritional components, for example protein or fats, using stepwise regression analysis. Preference patterns were experimentally corroborated by observations of captive animals. However, a lepidopteran herbivore which undergoes periodic outbreaks on Californian oaks, showed some degree of specialisation, although it was still susceptible to very high tannin concentrations. Variation in leaf chemistry was examined in relation to growth of larvae of oak moth, Phryganidia californica, on three deciduous and two evergreen oak species. A seasonal study of tannin polyphenols and nutritional components in the leaves of one evergreen and one deciduous oak species showed that, in both species, highest levels of tannin polyphenols occurred in newly expanding leaves and declined as the leaves aged, contrary to findings from other studies of oaks. The evidence from garrigue and maquis in southern France suggests that the chemical profile of plants in these shrublands is fairly plastic. Artificial soil fertilization increased the nutritional value of leaf material by increasing nitrogen content and decreasing condensed tannin concentration, while very heavy grazing pressure increased the levels of leaf polyphenols.