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This Paper reports experimental yields for polycultures of dry bean and tomato in Cost Rica. Two tomato population densities were planted in polycultures and monocultures. High overyielding (land equivalent ratio, LER = 2.41; P < 0.01) was observed in the polyculture treatment with high tomato planting density. There was a trend towards higher absolute tomato yields in the polycultures than in the monocultures. This suggests that bean production can be obtained from tomato fields without detrimentally affecting tomato yields. Because of tow overall tomato yields in these experiments, the results should tentatively be considered valid only under poor conditions. Possible mechanisms responsible for the observed overyielding are discussed including the Interference Production Principle nitrogen fixation and the moderating effects of plant diversity on pest and pathogen attack.