Bean yield at two sites in Costa Rica was affected by the interactions of maize and bush bean cultivars with cropping system (relay, row intercropping and sole cropping). The maize culti-vars were grouped according to leaf area index (LAI) and plant height. At a site with a severe weed problem, leafy maize of intermediate height allowed the most bean yield in relay but the least in intercropping. Tall, leafy maize allowed as much yield of intercropped beans as short, less leafy cultivars. In intercrops, tall maize plants shaded the beans less, while in relay crops leafy plants of intermediate height shaded and suppressed weed growth so benefiting the following bean crop. At the other site, all maize groups had the same effect on beans whether grown in relay or intercropped. Leafy maize cultivars of medium height are preferable for sole crops and for relay cropping of beans at physiological maturity when weeds are a problem, but less leafy types are better for bean intercropping or for relay crops where the beans are planted before maize maturity. Plant types selected for sole cropping are not necessarily best for poly-cultures and one crop may affect another even when their periods of active growth do not overlap.