Pharmacokinetics study of ceftiofur crystalline free acid (CCFA) was conducted in 14 adult captive smooth dogfish (Mustelus canis). A single dose of CCFA at 6.6 mg/kg was administered intramuscularly. Blood samples were collected prior to treatment and at 1, 2, 6, 12, 24, 32, 48, 72, 96, 120, 144, and 168 hr posttreatment. Naïve pooling of data from four sharks was used to generate the average plasma drug concentration at each time point. After concluding the study, additional blood samples were opportunistically collected from five randomly selected sharks at 1,920 hr. Plasma ceftiofur and desfuroylceftiofur metabolite concentrations were determined using reversed-phase high performance liquid chromatography (HPLC). Pharmacokinetic analysis was performed using a noncompartmental technique. Peak plasma concentration (C_max) was 3.75 µg/ml with a time to Cmax (T_max) of 96 hr. Ceftiofur plasma concentrations were maintained above 2 µg/ml for at least 168 hr and were still quantifiable at 1,920 hr.