The fall armyworm,Spodoptera frugiperda(J. E. Smith), a polyphagous pest of grasses and legumes, showed higher rates of cannibalism when feeding on red kidney bean seedlings than on corn seedlings. These differences are related to behavioral preferences. In choice tests, the larvae showed a strong preference for corn. To see if these differences in cannibalism rates could be attributed to host chemistry alone, these tests were repeated using detached corn and bean leaf disks. No differences in cannibalism rates were observed under these conditions, although cannibalism on corn was higher than in earlier experiments. In no-choice tests, the efficiency of conversion of ingested food (ECI) was higher on corn seedling foliage than on bean seedling foliage. The ECI of cannibalizing larvae that were limited to other larvae as a food source was equivalent to that on corn. Larvae were able to offset these differences in food quality to some degree by increasing relative consumption rates on beans. The relative growth rate was highest on corn, second highest on other larvae, and lowest on beans. These data provide the first nutritional indices for a normally herbivorous species on a carnivorous diet, and so are useful for ecological studies of between-species comparisons relating growth to food-source differences.