despite annual legume pasture are of great importance for dryland agricultural systems in mediterranean environments, there are few studies of n2 biological fixation (nbf) reported in chile. in this study the nbf of four annual legume species: subterranean clover (trifolium subterraneum l.), yellow serradella (ornithopus compressus l.), arrow-leaf clover (t. vesiculosum l.), and crimson clover (t. incarnatum l.) (experiment 1), as well as seven mixtures of these species (experiment 2) were assessed. the nbf was measured by the 15n natural abundance technique. the objective was to determine nbf in the legume species and in distinct mixtures used. the study was carried out in an andisol of the andean precordillera located in the humid mediterranean zone of chile. pasture was evaluated for biomass; and total n and natural abundance of 15n were analyzed in plant material samples. in experiment 1 (monospecific legume species pasture), n derived from fixation ranged between 43 and 147 kg n ha-1 and where t. vesiculosum and t. subterraneum presented statistical differences (p ≤ 0.05) in connection with the other species. in the legume mixtures (experiment 2), n derived by fixation varied between 97 and 214 kg n ha-1 where the 50-50 mixtures (t. subterraneum and o. compressus, or t. subterraneum and t. vesiculosum, respectively) had the highest n fixation. fixed n ranged between 12 and 25 kg n t-1 dm, showing significant differences among mono and polyspecific legume species.