The objective of this work was to study the operational feasibility of nitrification and denitrification processes in a mechanically stirred sequencing batch reactor (SBR) operated in batch and fed-batch mode. The reactor was equipped with a draft-tube to improve mass transfer and contained dispersed (aerobic) and granulated (anaerobic) biomass. The following reactor variables were adjusted: aeration time during the nitrification step; dissolved oxygen concentration, feed time defining batch and fedbatch phases, concentration of external carbon source used as electron donor during the denitrification stage and volumetric ammonium nitrogen load in the influent.The reactor (5 L volume) was maintained at 30±1ºC andtreated either 1.0 or 1.5 L wastewater in 8-h cycles.Ammonium nitrogen concentrations assessed were: 50(condition 1) and 100 mgN-NH4 +·L–1 (condition 2), resultingin 29 and 67 mgN-NH4 +·L-1·d–1, respectively. A syntheticmedium and ethanol were used as external carbon sources (ECS). Total nitrogen removal efficiencies were 94.4 and 95.9% when the reactor was operated under conditions 1 and 2, respectively. Low nitrite (0.2 and 0.3 mgN-NO2 –·L–1, respectively) and nitrate (0.01 and 0.3 mgN-NO3 –·L–1, respectively) concentrations were detected in the effluent and ammonium nitrogen removal efficiencies were 97.6% and 99.6% under conditions 1 and 2, respectively.