A derepressed R factor, RY2drd2, was transferred at a frequency of 5x10minus 3 between two strains of Yersinia enterocolitica mated on a membrane. Under the same conditions transfer of this R factor from Escherichia coli to Y. enterocolitica was observed at a frequency of only 7-7x10minus 6. This frequency was greatly increased when the recipient strain was heat-treated before mating. Heat exposure for optimum fertility was 50 to 52 degrees C for a period of 2 to 3 min. Mutants of Y. enterocolitica were isolated which were infected by RY2drd2 from E. coli or from Y. enterocolitica at the same frequency. These observations strongly suggest that a DNA restriction and modification system in Y. enterocolitica causes its low recipient ability for plasmids from other species.