Technical Abstract: The enrichment procedures used to isolate Yersinia enterocolitica may induce the loss of the virulent plasmid from the organism. Plasmid loss would result in the loss of virulence and the concomitant disappearance of the associated phenotypic characteristics. This report presents information on the effect of five current enrichment procedures including trypticase soy broth, phosphate buffered saline, peptone enriched phosphate buffered saline, phosphate buffered saline with sorbitol, and bile salts and MacConkey broth on plasmid stability. Virulence assays using crystal violet binding, low-calcium response, Congo red uptake, hydrophobicity by latex particle agglutination, and autoagglutination as well as plasmid DNA analysis showed that Y. enterocolitica did not lose the virulence plasmid during isolation at 28 C and the cells were still virulent. Moreover, post-enrichment treatment with alkali to reduce competing microflora, when used in conjunction with these five enrichment techniques, did not cause the loss of the virulence plasmid.