The effect of vegetational diversity on ants and herbivores was studied in a maize-bean(Zea mays-Phaseolus vulgaris)biculture and a maize monoculture in Nicaragua. Three basic hypotheses were examined: (1) the abundance of herbivores on maize would be less in the biculture than in the monoculture, (2) ant foraging activity would be higher in the biculture, and (3)the effect of ants as natural enemies of maize pests would account for differences in the abundance of those pests in the bicultures. The effects of plant diversity on the abundance ofSpodoptera frugiperda(J. E. Smith), the fall armyworm, andDalbulus maidis(De Long&Wolcott), the corn leafhopper, were examined. Ant-foraging activity was estimated as the proportion of tunafish baits occupied by ants in monocultures and bicultures. The effect of ants on the maize herbivores was investigated by excluding ants through selective poisoning. Cropping patterns significantly influenced the numbers of fall armyworm and corn leafhopper, but in different ways. The corn leafhopper was more abundant in bicultures, and the fall armyworm was more abundant in monocultures. No significant difference was found in the proportion of baits occupied by ants in maize monocultures and maize- bean bicultures. Ants had a significant effect on the two main maize pests,S. frugiperdaand D.maidis. There were significantly higher numbers of both pests in plots where ant foraging activity was reduced with insecticide. These results suggest the possibility of using ants as agents of biological control in the maize agroecosystem in Nicaragua.