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Plasmid genotypes of root nodulating rhizobial isolates of Sesbania, sampled from six ecologically distinct habitats, were characterized. Plasmid profile analysis revealed nine different plasmid types having molecular masses ranging from 30 to 300 MDa, distributed among six profile types that grouped the isolates into six plasmid classes. The six plasmid profiles were diverged from each other and lack many common plasmid types among them. Variation in number and types of symbiotic (Sym) plasmid was assessed by hybridization of plasmid profiles with sym gene probes. Relatedness among different plasmid types was assessed by hybridization of total DNAs as well as plasmid profiles of different isolates with labelled intact plasmid. Plasticity of plasmid genotype and possible recombination between different plasmid types is suggested from the results obtained. Structural diversity among sym plasmids was assessed by PCR amplified product profiles using primer corresponding to the reiterated nif promoter consensus element (NPC-PCR). A total of 26 NPC-PCR profile types were recognized. Genetic diversity among sym plasmids of isolates belonging to the same plasmid class and having similar sym plasmid suggested recombinations and rearrangements of sequences within the sym plasmids. Cluster analysis based upon similarity among profile types sorted the isolates across the ecological gradient. We suggest that habitat heterogeneity and plasticity of plasmid genotype together contribute for the generation of genetic diversity leading to strainal differentiation in rhizobia. (C) 2001 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.