The density and effectiveness ofRhizobium trifolii in six experimental areas were investigated. The densities of the nodule organism ranged from 100 cells per g soil at pH 3.5 to 1.8×10⁵ at pH 5.7. There was a high correlation between density and soil pH. In the open mountain areas the densities and overall levels of symbiotic effectiveness were lower than in the enclosed areas at a lower elevation. The results are given of the densities and effectiveness of nodule isolates on the plots of an inoculation trial two years after sowing. The effect of the inoculation and the liming treatments were still apparent. These results of Rhizobium strain distribution are discussed in relation to its possible effect on the contribution of white clover to a sown sward.