The genetic basis of symbiosis, mucoid structure and melanin biosynthesis in a local isolate of Rhizobium cicer was investigated. The strain was avery effective symbiont and produced substantial amounts of exopolysaccharides and melanin. It harbored three high-molar-mass plasmids of 55, 80 and 130 kb, respectively. Thirty-one melanin production-negative (Mep(-)) derivatives were obtained through plasmid curing. The plasmid patterns of cured derivatives indicated that the loss of 55 and/or 80 kb plasmids was sufficient for obtaining Mep- phenotype. The specific involvement of 55-kb plasmid in melanin production was confirmed upon re-introduction of this plasmid into one of the Mep- derivatives. Further investigation also indicated that the 130-kb plasmid might be necessary for both mucoidal appearance and symbiotic functions.