Drug-sensitivity of 70 strains of Y. enterocolitica and 24 of Y. pseudotuberculosis including reference strains and isolates from men or animals were determined by the agar plate-dilution method. All the strains of Y. enterocolitica were relatively resistant to penicillins and cephalosporins, while all of Y. pseudotuberculosis were sensitive to both of the agents. Most of the strains of both species excluding 13 resistant ones were sensitive to streptomycin. The resistant strains, however, were sensitive to other aminoglycoside antibiotics, i.e. kanamycin, paromomycin, gentamicin, tobramycin, dibekacin and amikacin. All of the strains were sensitive to chloramphenicol. Most of the strains excluding 3 resistant ones were highly sensitive to tetracycline. The resistant strains were also resistant to doxycycline and methacycline but not to minocycline. All of them were resistant to erythromycin, to lincomycin and to novobiocin. Most of them excluding 7 resistant strains were sensitive to sulfisoxazole. All of them were sensitive to nalidixic acid, piromidic acid, and also to dihydroxymethylfuratrizine. A new synthetic agent, fosfomycin showed a relatively wide range of activity to the strains, but none of resistant strains were noticed. These resistant strains were all found in the isolates from men and animals but not in reference strains. Among these resistant strains, resistance patterns were as follows; TC, SM, SA in 3 of Y. enterocolitica, SM, SA in 3 of Y. enterocolitica and in 1 of Y. pseudotuberculosis, and SM in 3 of the both species, respectively. As for polypeptide antibiotics, polymyxin B and colistin, some strains of the both species showed an uncontinous susceptibility, i.e. inhibited growth at a certain lower concentration but growth at a higher concentration.