In Latin America there are millions of hectares where the agriculture is limited by managerial, nutritional and environmental factors and where well adapted leguminous plants can contribute to increased production for the food and cattle raising industries. The low availability of phosphorus is one of the main limitations for the development of pastures in andisolic soils, where a high capacity for the fixation of this nutrient exists and where chemical fertilization temporarily meets plant requirements. The objective of this study was to select an arbuscular mycorrhizal fungus (AM) for its ability to improve the yield and to stimulate symbiotic phosphorus fixation in a forage legume, Lucerne ( Medicago sativa L.): variety AS-13 for a soil, Andic dystropept. The effects of the inoculation of 5 AM fungi strains on dry weight yields and the phosphorus content in the plant shoots and the percentage of infection in the root were greenhouse evaluated, in tyndalized and non-tyndalized soil.