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A series of bench-scale nitrification/denitrification tests were carried out with both suspended growth and hybrid bioreactors. The hybrid reactor was filled with plastic (polyethylene) media to evaluate the effects of biofilm. Two types of reactor configurations were tested; 4-compartment and 6-compartment modes. The experiments were initiated with a half-strength pulp and paper wastewater and its strength increased stepwise to the raw wastewater. Solid retention time was fixed at 10 days after a start-up period while hydraulic retention time was extensively varied from 3.5 to 0.5 days. The results from each type of reactor were compared in terms of nitrification/denitrification efficiency and stability. Experimental results demonstrate that the hybrid system showed greater stability in nitrification and higher denitrification efficiency than the suspended growth system. In the hybrid system, attached volatile solids formed 61- 72% of total volatile solids in the reactor and the amount of attached volatile solids insignificantly varied with the organic loading rate (0.37 - 2.76 kg COD M(-3) d(-1)) after initial biomass attachment. Under the conditions tested (0.1 - 2.8 kg COD m(-3) d(-1)), organic loading rate insignificantly influenced the nitrification. Better performance was obtained in denitrification when the anoxic zone was better isolated from the aerobic compartments (6-compartment mode). Overall, the hybrid system with fixed-film growth had better resistance to upset caused by transients such as changes in influent composition or hydraulic retention time.