Symbiotic nitrogen fixation in legumes reaches its optimum when the host plants are nodulated by highly competitive and effective Rhizoblum strains. With the purpose of assessing competition and nitrogen fixing capacity of native Rhizoblum leguminosarum biovar trifolli strains, a greenhouse test was carried out using white clover (Trifollum repens) and four kinds of soils, wich represent the main agroecological areas of the IX Region. Eleven strains were evaluated, they were both native and collected and were streptomycin sulphate antibiotic resistant. A nitrogen and a nitrogenless without inoculation testers were used as controls. AII pots received a solution of ammonium sulphate marked with 10% 15N a.e., equivalent to 10 kg ha-1 of N. Ryegrass was used as reference crop, cv. Nui. In general, the evaluated strains were very efficient. After three or four cuts they became the only source of nitrogen for the plants. They were also very competitive, getting to over 70% of root infection with regard to those present in soils. In Curacautin and Toltén soils, biological nitrogen fixation begins later than the other soils evaluated, which is coincident with soils having a higher nitrogen content. Symbiosis occurs when the soil nitrogen content exhausts or diminishes.