The pharmacokinetics of erythromycin (EM) after oral administration was studied in the cultured olive flounder, Paralichthys olivaceus, using LC/MS/MS. After single- or multiple-dose administration of EM (50, 100, 200 mg/kg body weight and 50 mg/kg for 5 days) by oral route in olive flounder ($350{pm}40g$, $22{pm}0.5^{circ}C$), the concentration in the serum was determined at 1, 3, 6, 9, 24, 72, 120, 168, 264, 360, 504 and 720 h post-dose. The kinetic profile of absorption, distribution and elimination of EM in serum were analyzed fitting to a two-compartment model by WinNonlin program. The area under the concentration-time curve (AUC), maximum concentration ($C_{max}$), time for maximum concentration ($T_{max}$) following oral administration of 50, 100 and 200 mg/kg b.w. and 50 mg for 5 days. EM was $165.3hr^*{mu}g/m{ell}$ ($C_{max}$, $34.63{mu}g/m{ell}$; $T_{max}$, 1.56 hr), $212.8hr^*{mu}g/m{ell}$ ($C_{max}$, $60.38{mu}g/m{ell}$; $T_{max}$, 3.99 hr), and $592.37hr^*{mu}g/m{ell}$ ($C_{max}$, $63.01{mu}g/m{ell}$; $T_{max}$, 4 hr), respectively. The results of this study related to dosage and ${mu}{cdot}$withdrawal times could be used for prescription of EM in field for the treatment of bacterial diseases in olive flounder.