Three strains of Rhizobium leguminosarum bv. phaseoli were tested with three bean varieties, in two different Costa Rica communities: Los Chiles and Puriscal. At Puriscal, the strain CR-477 (a native Costa Rica strain) showed a greater development (nodule dry weight, tissue dry weight and nitrogen content) and higher yields, when used with the bean variety Talamanca. Even greater than the nitrogen fertilizer control. Similar results were showed by the varieties Criolla and Brunca, at Los Chiles. The results showed that with these varieties, the local strain (CR-477) gave better results than strains from CIAT and the chemical fertillizer application (150 kg/ha). The use of inoculants with highly effective strains has shown to have a large potential for yield improvement, at a lower production cost by reducing the amount of fertilizer used.