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The use of leguminous plants in the agricultural systems of the humid tropics is an alternative means for providing N to soils considered unfertile and marginal. The objective of this study was to assess the biological nitrogen fixation (BNF) in the symbiosis Rhizobia-herbaceous legumes (Mucuna deeringiana, Canavalia ensiforinis, Vigna umbellata, Cajanus cajan, Dolichos lablab, Arachis pintoi and Crotalaria juncea) grown in acid soil amended with P fertilizer by the 15N-isotopic dilution technique ("A"-value) using maize, Brachiaria spp. and no nodulating common bean as reference crops.. Results indicate a positive effect of P-fertilizer on parameters associated with BNF and biomass accumulation. The legumes Cajanus cajan and Canavalia ensiformis showed good performance in terms of dry weight of nodules (C. cajan = 758 and C'. ensiformis = 500mg nodules plant-1 with 49 and 56 nodules plant-1, respectively) at flowering stage, and BNF (C. cajan = 170 and C.ensiformis = 366 kg N-1 cycle-1) when 80kg P2O5 ha-1 was applied to soil. Maize 'VS-486' and 'VS-536' and non-nodulating common bean 'DOR 364' were adequate reference crops to estimate the N derived from the atmosphere by the legume species. The results indicate that C. ensiforinis and C.cajan have a good potential to be used as green manure in the Huimanguillo savannah, Tabasco State, Mexico. [ABSTRACT FROM AUTHOR]