The treatability of pharmaceutical industry wastewater bearing the antibiotic sulfamerazine in the lab-scale continuously feed upflow anaerobic sludge blanket (UASB) reactor/continuously stirred tank reactor (CSTR) system was investigated in order to determine the changes in the characterization on composition of the wastewater during biotreatment. The BOD5/COD ratio was determined to assess the approximate biodegradability. When the initial sulfamerazine concentration was 10 mg/l the methane percentage reached around 76% while it was measured at 60% at a sulfamerazine concentration of 90 mg/l. The total (overall) COD removal efficiency of the sequential UASB + CSTR treatment system was determined as 97%. The results of this study indicated that the system exhibited a good removal performance for sulfamerazine. (c) 2006 Elsevier B.V. All rights reserved.