Sulfamethazine (SMZ) is an antibiotic from sulfonamides class widely used in veterinary medicine and reported in wastewater and sewage. Thus, it is essential to study technologies to reduce SMZ present in the aquatic environment. Anaerobic bioreactors are a low-cost technology applied for wastewater treatment. The objective of this paper is to study kinetics parameters related to SMZ removal using a horizontal flow-anaerobic immobilized biomass reactor (HAIR) and to evaluate its transformation products formed during this treatment. The bioreactor was operated at mesophilic condition with a hydraulic retention time of 12 h. The removal of SMZ was evaluated at three different concentrations: 200 ng L-1 (phase I), 400 ng L-1 (phase II) and 600 ng L-1 (phase III). The apparent first-order removal constant obtained for chemical oxygen demand was 0.885 +/- 0.094 h(-1) while SMZ showed a removal constant of 0.356 h(-1). SMZ was removed with an efficiency of 56.0 +/- 13.0 % (phase I); 62.0 +/- 12.0 % (phase II) and 62.0 +/- 6.00 % (phase III). Seven transformation products were detected and one of these with m/z 233 is reported for the first-time. The HAIB bioreactor has a potential to assist in wastewater treatment to remove contaminants at ng L-1 concentration level.