Ineffective nodulation in the soybean [Glycine max (L.) Merrill] inoculated with Rhizobium japonicum strain 61 was shown to be controlled by a single dominant gene which we have designated Rj4.

Ineffective nodulation in soybeans (Glycine max (L.) Merrill) inoculated with Rhizobium japonicum strain 33 was found to be controlled by a single dominant gene which has been designated Rj3.

Soybean varieties (Glycine max (L.) Merrill) were inoculated with 28 strains and two commercial preparations of Rhizobium japonicum. The effect of each strain on the yield of five varieties was evaluated and significant yield differences were found associated with certain strains. Four different environments were studied and in three of the environments, which were free of R. japonicum, a significant strain ✕ year interaction was observed. No significant strain ✕ variety interactions were indicated. Only 5 to 10% of the nodules on plants grown in a field with an established R. japonicum population were derived from the inoculum introduced into the soil on the seed.

Soybean genotypes grown successive years in different fields were predominantly nodulated by Rhizobium japonicum strains of specific scrogroups. Chi‐square analyses showed significant differences among plant genotypes in their acceptance of Rhizobium japonicum strains of specific serogroups. Closely related soybean genotypes had similar distributions of R. japonicum in their nodules. The results indicate specific interactions between soybean genotypes and R. japonicum serogroups hi nodule formation.