The pasture-breeding program to improve production in the natural grasslands in Uruguay has acknowledged that indigenous Rhizobium strains are incompatible with introduced Mediterranean clovers. In an attempt to understand and overcome this problem, a cross-row experiment was set up in 1999 in a basaltic, acid soil in Glencoe, Uruguay, to follow the survival and performance of 9 exotic strains of Rhizobium leguminosarum bv. trifolii. This paper reports on the ability of the introduced strains to compete for nodule occupancy of Mediterranean clover hosts and impacts of the introduced strains on the productivity of the indigenous Uruguayan clover Trifolium polymorphum. Strain WSM1325 was a superior inoculant and remained highly persistent and competitive for the effective symbiosis with the Mediterranean hosts, T. purpureum and T. repens, in the Uruguayan environment in the third year of the experiment. The Mediterranean hosts (T. purpureum and T. repens) nodulated with the introduced strains but did not nodulate with any indigenous R. leguminosarum bv. trifolii typed from nodules of T. polymorphum. Conversely, there were no nodules on the Uruguayan host T. polymorphum that contained introduced R. leguminosarum bv. trifolii. These results reveal the establishment of effective symbioses between strains of R. leguminosarum bv. trifolii and clover even though the soil contained ineffective R. leguminosarum bv. trifolii for all hosts. We believe our results are the first reported example of ‘selective’ nodulation for an effective symbiosis in situ with annual and perennial clovers in acid soils.