Yersinia pestis is the causative agent of plague, a disease which has caused the deaths of millions of people and which persists now in endemic foci. The rRNA gene restriction patterns (i.e., ribotypes) of 70 strains of Y. pestis, isolated on the five continents over a period of 72 years, were determined by hybridization with a 16S-23S rRNA probe from Escherichia coli. The combination of the EcoRI and EcoRV patterns resulted in the elucidation of 16 ribotypes. Two of them (B and O) characterized 65.7% of the strains studied, while the 14 other ribotypes were found in no more than three strains each. A relationship was established between biovars and ribotypes: strains of biovar Orientalis were of ribotypes A to G, those of biovar Antiqua were of ribotypes F to O, and those of biovar Medievalis were of ribotypes O and P. Great heterogeneity in rRNA restriction patterns was found among strains isolated in Africa; this heterogeneity was less pronounced among Asian isolates and was completely absent from the American strains. Pulsed-field gel electrophoresis was performed on the DNAs of some strains, but it appeared that different colonies from the same strain displayed different pulsed-field gel electrophoresis patterns and therefore that this technique was not suitable for comparison of Y. pestis isolates. In contrast, the ribotypes of individual colonies within a given strain were stable and were not modified after five passages in vivo. A clear correlation between the history of the three plague pandemics and the ribotypes of the strains could be established.