Seven effective (N2-fixing) strains of R. meliloti were studied. By sedimentation analysis of their alkaline lysates in alkaline sucrose gradients, a plasmid was found in 4 strains. In a strain (2011 str 3) which gave no result with this method, supercoiled DNA was detected by CsCl-dye buoyant density gradient centrifugation. That result was confirmed by analytical Cs2SO4-Ag+ density gradients, which showed a heterogeneity in the average base composition of the DNA extracted from 3 strains, including the 2011 str 3 strain. Two of those last strains seemed to contain an extrachromosomal DNA of very high MW.