Leucaena species are leguminous plants native to Mexico. Using two L. leucocephala cultivars grown in different soils, we obtained 150 isolates from the nodules. Twelve rDNA types were identified which clustered into groups corresponding to Mesorhizobium, Rhizobium, and Sinorhizobium by restriction fragment length polymorphism (RFLP) of amplified 16S rRNA genes. Types 2, 4, 5, 6, 10, 11, and 12 were distinct from all the defined species. Others had patterns indistinguishable from some recognized species. Most of the isolates corresponded to Sinorhizobium. Forty-one electrophoretic types (ETs) were identified among the isolates based on the different combinations of electrophoretic patterns of 13 metabolic enzymes. ETs were clustered into groups in general agreement with the rDNA types. Diverse plasmid patterns were obtained among the isolates, but common plasmids were observed among most isolates within rDNA types 5, 10, and 11. The symbiotic plasmids were identified among most of the isolates, except for the Mesorhizobium isolates. The affinities of host cultivars for different rhizobial groups and the impact of soil cultivation on the soil populations of rhizobia were analysed from the estimation of isolation frequencies and diversity. The results showed differences in rhizobial populations in cultivated and uncultivated soils and also differences in rhizobia trapped by L. leucocephala cv. Cunningham or Peruvian.