Heat-shock treatment of Rhizobium meliloti cells causes major enhancement in the synthesis of several proteins with apparent molecular weights in the range of 58-60 kDa. Using the polymerase chain reaction and degenerate oligodeoxyribonucleotide primers for conserved regions of the 60-kDa heat-shock protein (HSP60) or GroEL protein family, a 0.6-kb probe for the R. meliloti hsp60 gene was prepared. Southern blot analysis of R. meliloti DNA digested with different restriction enzymes and hybridized to R. meliloti hsp60 probes indicated the presence of between four and five hsp60 or groEL in this species. From the cloning and sequencing of several of these fragments, we have been able to deduce the complete nucleotide sequences of three groEL in R. meliloti. The deduced amino acid (aa) sequences of these proteins show extensive similarity to each other (78-85% aa identity) and to other GroEL homologues. In the upstream regions of two of the groEL, but not the third, open reading frames corresponding to GroES proteins were also identified. Analysis of various prokaryotic GroEL sequences suggests that the multiple groEL of R. meliloti have evolved by means of gene duplication events within this or a related group of organisms. Results presented in this paper also show that some of the groEL in R. meliloti are located on the two megaplasmids present in these cells. The presence of multiple GroEL homologues in R. meliloti suggests a possible role of the GroEL or HSP60 chaperonins in the nodulation (symbiosis) and nitrogen fixation processes.