Strains and numbers of Rhizobium japonicum in the soil can influence soybean (Glycine max (L.) Merrill) yields but R. japonicum serogroups and numbers in Ontario soils have not been studied. In 1976 and 1977, the influences of inoculation and fertilizer nitrogen (N) levels on populations, measured by most probable number (MPN) techniques, and serogroups of R. japonicum were studied at three Ontario sites. The locations, near Ridgetown, Elora, and Woodstock, represented areas where soybeans had been grown many times previously, only a few times or not at all, respectively. Treatments were (a) un inoculated + O N; (b–e) inoculated +0, 50, 100 or 200 kg N/ha.At Ridgetown, MPN's, expressed on log10 bases, ranged from 5.17/g soil in 1977 to 7.02 in 1976. At Elora, comparable values were from 3.66 to 6.43. Inoculation did not increase MPN's at either site. At Woodstock, inoculation increased MPN's from 0 to about 2.47 in both years. Nitrogen application did not change the MPN values at any site in either year.Serogroup 110 predominated in the nodules at Ridgetown in both years. At Elora, serogroup 123 was found most frequently in nodules. Serogroups 311b 138 in 1976 and 122 and 125 in 1977 were found most frequently after inoculation at Woodstock. Additions of fertilizer N had only slight effects on the distribution of serogroups in the nodules.