Studies are being made to apply ampicillin for pseudotuberculosis of cultured yellowtails. In vitro sensitivity and development of drug-resistance characteristics of Pasteurella piscicida were examined. Chloramphenicol was used for comparison. The minimum inhibitory concentration of ampicillin to P. piscicida was 0.04 to 0.08 mcg/ml and the MIC of chloramphenicol was 0.63 mcg/ml. Antibacterial activity of ampicillin was 8 to 16 times greater than the activity of chloramphenicol. After 10 serial passages in vitro under the presence of drug, MIC values to ampicillin and to chloramphenicol were 4 to 8 times greater than the initial values. Reversion of acquired drug-resistance features to ampicillin also was studied. After 5 serial passages in vitro in BHI agar under the absence of the drug, the acquired drug-resistance characteristics of P. piscicida disappeared, i.e., MIC changing from 0.32 mcg/ml to 0.04-0.08 cmg/ml. Results of the study suggest that ampicillin is an useful antibacterial drug for pseudotuberculosis of cultured yellowtails.