The effects of intraplant variation of nitrogen, phosphorus, potassium and wafer content within maize plants on fitness of the African armyworm, Spodoptera exempta (Walker), were studied in three experiments by using an excised leaf system. Maize plants were phosphorus deficient in one of the experiments. Various leaf categories were distinguished differing in age and position on the plants. Leaf sections on which caterpillars fed were daily refreshed. Changes in the four plant variables measured, due to the excision and subsequent placement of the leaf sections in the excised leaf system for one day, were small and mostly insignificant. Highly significant differences were found between the composition of the various leaf categories in all three experiments. In all experiments, the nitrogen concentrations ranged from 2.4 to 4.5%, phosphorus from 0.1 to 1.3%, and potassium from 1.4 to 4.8% dry weight, while water content varied from 83 to 90% fresh weight. Of all insect fitness parameters observed, only larval growth rate was consistently and significantly affected by leaf category. Fecundity was strongly correlated with pharate adult weight. Only phosphorus was significantly and positively correlated with larval fresh weight, especially in the experiment with phosphorus deficient plants. Multiple regression models revealed that in all experiments a significant proportion of variation in larval fresh weight between leaf categories could be explained by the four plant variables. However, only phosphorus was included in all models. When all leaf categories used in the three experiments were considered simultaneously, the importance of phosphorus for larval growth rate in these experiments was again evident. The variation in nitrogen, potassium and water appeared to be of little or no significance. The consequences of these findings for outbreak development of S. exempta are discussed.