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White clover plants grown at a site contaminated with heavy metals following applications of sewage sludge were found to have small white root nodules containing ineffective rhizobia (S isolates) which had identical plasmid profiles, unlike the diverse profiles of effective rhizobia from root nodules on adjacent control plots. Our paper supports an earlier suggestion that the ineffective S isolates of Rhizobium from nodules of white clover grown on heavy-metal contaminated soil represent a single strain. These new data include restriction fragment length polymorphism (RFLP) studies using probes specific for a chromosomally-located gene (lac), a plasmid-located symbiotic gene (nifH,D) and a repeated sequence specific for Rhizobium leguminosarum bv. trifolii (RtRS). RFLP patterns of isolates from control plots indicated that although these strains showed variation, they were related to one another but not to the S …