Plasmid content in 242 Yersinia pestis strains from various natural plague foci of the U.S.S.R. and other coutries was studied. Of these strains, 172 (71%) were shown to carry three plasmids described previously of about 6, 45–50 and 60 MDa, respectively. Twenty strains (8%) from different foci harboured additional cryptic plasmids, most often of about 20 MDa in size. Plasmid pPst displayed considerable constancy of its molecular mass. On the contrary, size variations of pCad (45–49 MDa) and, especially, pFra (60–190 MDa) were found. Molecular mass of these plasmids correlated with the host strain origin.