||1. Factors affecting the nutritional ecology of mixed-feeding, polyphagous herbivores are poorly understood. Mixed-feeding herbivores do better when they consume both forb and grass species although they typically feed primarily on forbs, which are of relatively higher protein content than grasses. 2. In a field experiment, we examined the effects of nitrogen and phosphorus fertilization and associated changes in host-plant C:N:P on proportional grass consumption by a mixed-feeding insect herbivore, Melanoplus bivittatus, using natural abundance stable carbon isotope (C-12/C-13) methods. We also examined a grass-feeding (Phoetaliotes nebrascensis) and forb-feeding (Hesperotenix viridis) species. 3. The C isotope signatures of M. bivittatus collected from plots fertilized with nitrogen (+N), phosphorus (+P), nitrogen and phosphorus (+N+P) and no fertilizer were compared with the C isotope signatures of plants in those plots to determine the proportion of assimilated C derived from C-4 grasses and C-3 forbs in each plot. We also examined the relationship between M. bivittatus diets and plant C:N:P stoichiometry. 4. The proportion of grass assimilated approximately doubled in N-fertilized treatments (39.1 +/- 0.1%) compared with non-fertilized treatments (19 +/- <0.1%), an increase associated with decreased C:N and increased N:P of grasses. 5. These results indicate that mixed-feeding M. bivittatus can selectively feed to balance C:N:P intake even when choosing between two structurally and chemically different groups of plants. 6. The strong relationship between diet selection and grass stoichiometry also suggests that plant nutrient composition may be more important than defensive chemistry in food choice.