Seven strains of Y. enterocolitica, of which 2 strains being resistant to SM, TC and SA, 2 to SM and SA 3 to SM, and 3 strains of Y. pseudotuberculosis resistant to SM have been isolated from men and domestic animals. Tests were carried out with these 10 isolates for transfer of the drug resistance by bacterial conjugation, and evidence was obtained for transfer of resistance from 1 SM-resistant Y. enterocolitica and 2 Y. pseudotuberculosis strains to the sensitive E. coli K-12. Moreover, secondary transfer of this resistance to sensitive strains of the donor species Y. enterocolitica and Y pseudotuberculosis was also demonstrated. The findings indicate the spontaneous occurrence of organisms carrying the resistance transfer factor, R(Sm), among the populations of these two species in the natural environment.