A suspended-growth batch reactor was used to denitrify synthetic wastewater containing various proportions of nitrate and nitrite. A competitive phenomenon between nitrate- and nitrite-reductase was studied utilizing various proportions of nitrate and nitrite in an anaerobic environment with a temperature of 30 degrees C and methanol as carbon source. By using a non-linear regression technique, biokinetic constants of the maximum specific reduction rates of nitrate and nitrite (k(1), k(2)) and the Monod half-saturation coefficients of nitrate and nitrite (K-s1, K-s2) for the proposed two-step denitrifying kinetics were 1.29 day(-1), 0.89 day(-1) and 14.3 mg NO3--N dm(-3), 10.9 mg NO2--N dm(-3), respectively. The result obtained from a series of chemostat studies indicated the Monod-type kinetic model was more accurate when the distributed ratio of nitrate- and nitrite-reductase in the proposed two-step denitrifying kinetics was taken into account.