Abstract Host plant preference for nodulating strains and strain competitiveness within two natural soil populations of Rhizobium leguminosarum biovar trifolii were investigated using the techniques of polyacrylamide gel electrophoresis (PAGE) of isozymes and agarose gel electrophoresis of plasmid DNA. Four cultivars of white clover showed no differential preference for particular PAGE electrophoretic types (ETs). Nor was there preference for particular plasmid profile types, the high degree of variation in this character indicated strain heterogeneity within ET groupings. Intra-plant variation of ETs was representative of the population as a whole. Soil dilution series were used to test for competitiveness effects on nodulation. The proportion of the major nodulating ETs did not change with dilution, indicating no change in the competitiveness of ETs as a result of a decrease in cell concentration. This suggested that ETs nodulating in the majority occurred frequently in the soil inocula tested. Plasmid profile analysis also showed heterogeneity within ET groupings and indicated that no one strain had increased in competitiveness following dilution. Our data were used to examine the acceptability of these sampling and screening methods to study other rhizobial populations.