Nitrogen deficiency severely limits production of the bean-maize association grown by smallscale farmers in upland Peru. Sixty-four bean Rhizobium isolates were evaluated for induction of a plant growth response in pots of soil from the area and 14of the most promising strains of these were tested on farmers' fields in 13 trials over a three year period. Bean yields were increased by inoculation with at least one Rhizobium strain in seven of the trials, with increases ranging from 0.21 to 0.68 t ha" 1 . Bean yields were not increased by the application of 120 kg N ha" 1 to the intercrop. Maize yields were increased by Rhizobium inoculation in nine of the 13 trials by between 0.34 and 1.85 t ha" 1 . Maize responded to nitrogen fertilizer on the same nine farms. This Rhizobium selection strategy, although laborious, was effective in identifying strains that can be recommended for use as inoculants by farmers in the region.Inoculation de Rhizobium en una siembra simultdnea deporotolmaiz RESUMEN La deficiencia de nitrogeno limita severamente la production en el cultivo asociado de poroto/ mafz empleado por los agricultores en pequena escala de las tierras altas del Peru. Se evaluo la induction de una respuesta de desarrollo de planta utilizando 64 aislados de Rhizobium de poroto en macetas con tierra de la region, y 14 de las cepas mas prometedores fueron puestas a prueba en los campos de los agricultores en 13 ensayos realizados durante el transcurso de 3 afios. En 7 ensayos los rendimientos de poroto aumentaron con la inoculation de por lo menos una cepa de Rhizobium, con incrementos de entre 0,21 y 0,68 t ha" 1 . Los rendimientos de poroto no aumentaron mediante la aplicacion de 120 kg N ha" 1 al cultivo simultaneo. El mafz respondio al fertilizante con nitrogeno en las mismas 9 fincas. Esta estrategia de selection de Rhizobium, si bien laboriosa, resulto eficaz para identificar las cepas que se pueden recomendar como inoculantes a los agricultores de la region.