Instituto de Investigaciones Agropecuarias y Forestales. Instituto de Investigaciones sobre los Recursos Naturales. Instituto de Investigaciones Químico Biológicas. Facultad de Biología. Facultad de Medicina Veterinaria y Zootecnia. Facultad de Químico Farmacobiología. Programa Institucional de Doctorado en Ciencias Biológicas ; Companion planting is an alternative to maintaining and improving soil fertility, as the biological fixation of nitrogen is the cheapest source of N for agricultural acid soils of tropical savannahs. The objective of this research was to quantify biological fixation of nitrogen by Fabaceae and contribution of N to maize (Zea mays L.). Specific objectives were to evaluate the performance of 32 genotypes of maize and response to N fertilization, measure the amount of fixed N in four species of Fabaceae through the 15 N isotope dilution technique and evaluate maize in association and rotation with Fabaceae in acid soils of the savannah of Huimanguillo, Mexico. Three experiments were set up with a design of a randomized complete block, where the treatments were maize genotypes with four replications. For the first experiment were evaluated; dry matter production (MS), grain yield (GY) and efficiency of utilization of 15 N- fertilizer (EUNF). Variables evaluated for the second experiment were green manure production (AV), dry matter production (DM), number of nodules (NN), dry mass production of nodes (MSN), Nt and NF. Variables in the third experiment in association and rotation for maize (CML269, HQ4-C, Creole and S03) were dry matter (organs and total), grain yield and Nt, and to the Fabaceae (M. deerengiana L. and C. cajan L.) were AV, MS, number and dry mass nodules, Nt and NF. ; La asociación de cultivos es una alternativa para mantener y mejorar la fertilidad de los suelos, ya que la fijación biológica de nitrógeno es la fuente de N más barata y viable para los suelos ácidos de aéreas agrícolas del trópico húmedo. Como objetivo general de esta investigación fue cuantificar la fijación ...