Rhizobium leguminosarum by. trifolii (Rlt) establishes beneficial root nodule symbiosis with clover. Twenty Rlt strains differentially marked with antibiotic-resistance markers were investigated in terms of their competitiveness and plant growth promotion in mixed inoculation of clover in laboratory experiments. The results showed that the studied strains essentially differed in competition ability. These differences seem not to be dependent on bacterial multiplication in the vicinity of roots, but rather on complex physiological traits that affect competitiveness. The most remarkable result of this study is that almost half of the total number of the sampled nodules was colonized by more than one strain. The data suggest that multi-strain model of nodule colonization is common in Rhizobium-legume symbiosis and reflects the diversity ofrhizobial population living in the rhizosphere.