Abstract: This paper studied the pharmacokinetics of difloxac (DIF) in grass carps (Ctenopharyngodon idellus) following a single 20 mg/kg oral dose at 15℃. The concentrations of DIF in blood and tissues (muscle, liver, and kidney) were detected by HPLC. The parameters were analyzed based on the classical compartmental analysis. The results showed that the DIF were rapidly absorbed by the grass carp and widely distributed in the blood and tissues. The drug concentration-time curves of DIF in blood and tissues all were best described by a two compartment-open model. And the best concentration-time equation of blood was C=5.056e^-0.012t+19.041e^-0.011t. In addition, the absorption half-life (T_1/2α) of DIF in the blood, muscle, liver and kidney were 0.176h, 0.562h, 4.562h, and 1.477h, respectively. The elimination half-life (T_1/2β) of DIF in the corresponding tissues were 69.492h, 65.303h, 218.412h, and 163.937h, respectively. The body clearances (CL) in the corresponding tissues were 0.495, 11.181, 10.789, and 7.102 L/h/kg, respectively. The corresponding areas under the curves (AUC) were 81.550, 1277.55, 807.470 and 1432.150 μg/L/h, respectively. What's more, the recommended withdrawal period of DIF was more than 26 days under the standard of not more than 100 μg/kg residual amount in the edible tissues.