The species relevance of atypical Yersinia strains was determined by various microbiological, immunological, and genetic (including polymerase chain reaction) tests. These strains were shown to represent mixed cultures of Y. pseudotuberculosis serovariant O1b and Y. pestis var antiqua. Identification-resistant cells with atypical properties and plasmid segregation were found in the populations of Y. pestis strains. Analysis of different diagnostic tests revealed the most reliable ones selected for the identification of atypical Y. pestis strains with unstable genome.