Residues of ampicillin and amoxicillin in the muscles of olive flounder Paralichthys olivaceus were investigated after oral doses of 20 mg/kg body weight/day of ampicillin and 40 mg/kg body weight/day of amoxicillin in a first examination and 40 mg/kg body weight/day of ampicillin and 80 mg/kg body weight/day of amoxicillin in a second examination for 5-6 days. The ranges of seawater temperatures in the 1^(st) and 2^(nd) examinations were 13.4-15.6℃and 16.8-21.4℃, respectively. Ampicillin and amoxicillin concentrations were analyzed by high performance liquid chromatography (HPLC). The average recovery rates of ampicillin and amoxicillin in fish samples ranged from 84.8-95.2% and 100.8-103.8%, respectively. Residual concentrations in the olive flounder peaked on the 5th day, with average concentrations of 0.075 mg/kg and 0.311 mg/kg in the 1st examination, and 0.098 mg/kg and 0.630 mg/kg in the 2^(nd) examination for ampicillin and amoxicillin, respectively, with maximum concentrations in muscle. In the 1^(st) examination, ampicillin concentrations in olive flounder after withdrawals of 1 and 3 days were 0.041 mg/kg and 0.023 mg/kg, respectively, and amoxicillin concentrations after withdrawals of 1 day and 10 days were 0.172 mg/kg and 0.023 mg/kg, respectively. In the 2^(nd) examination, the ampicillin concentrations in olive flounder after withdrawals of 1 and 3 days were 0.041 mg/kg and 0.023 mg/kg, respectively, and amoxicillin concentration after withdrawals of 1 day and 10 days were 0.172 mg/kg and 0.023 mg/kg, respectively. We suggest that the recommended withdrawal periods should be 3 days for ampicillin and 10 days for amoxicillin in the olive flounder.