The present research was carried out to study the selectivity of the mixture bentazon (3-isopropil-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide) plus paraquat (1,1'-dimethyl -4-4' -bipyridinium ion) on the bean varieties 'Carioca' and 'Moruna', the effect of the mixture on weed control, and the possible sinergistic effects on weeds. The experiment is being conducted at the experimental area of the Agriculture and Horticulture Department - ESALQ/USP - Piracicaba, and at the greenhouse. A randomized complete block design with four replications was used. The evaluation was done by percent of injury to the varieties and weeds, by visual scale. The data were analysed by the formula of Gowing (Colby, l). By this method, the injury observed on the varieties was lower less than expected, due to an antagonistic effect. Antagonistic effects were also observed with Digitaria sanguinalis (L.) Scop. The sinergistic effect was observed with Sida glaziovii K. Sch. and Portulacca oleracea mainly with the mixture of 0.48 + 0.05 kg a.i./ha of bentazon and paraquat respectively, at six and eight days after application. The mixture of 0.96 + 0.10 a.i./ha bentazon + paraquat presented an antagonistic effect for Cassia tora L.