Esterases of 32 strains of Y. enterocolitica and of 16 strains of Y. Pseudotuberculosis were compared by electrophoresis in polyacrylamide-agarose gel and by electrophoresis in a 4 to 30% polyacrylamide gradient gel. Differences observed in electrophoretic distributions and in molecular weights of esterases defined distinct esterase patterns for these two species.