The effect of dissolved oxygen (DO) and agitation rate in open and closed reactors was examined for sulfur-utilizing autotrophic denitrification. The reaction rate constants were determined based on a half-order kinetic model. Declining denitrification rate constants obtained for open reactors those of 8.46, 8.03, and 2.18 for 50 mg NO(3) (-)-N/L, while 11.12, 9.14, and 0.12 mg(1/2)/L(1/2) h were for 100 mg NO(3) (-)-N/L at agitation speeds of 0, 100, and 200 rpm. In closed reactors, the ever-increasing denitrification rates were 10.13, 22.56, and 37.03, whereas for the same nitrate concentrations and speeds the rates were 13.17, 15.63, and 26.67 mg(1/2)/L(1/2) h. The rate constants correlated well (r ( 2 ) = 0.89-0.99) with a half-order kinetic model. In open reactors, high SO(4) (2-)/N ratios (8.02-75.10) while in closed reactors comparatively low SO(4) (2-)/N ratios (6.10-13.39) were obtained. Sulfur oxidation occurred continuously in the presence of DO, resulting in mixed cultures acclimated to sulfur and nitrate. SO(4) (2-) was produced as an end product, which reduced alkalinity and lowered pH over time. Furthermore, DO inhibited sulfur denitrification in open reactors, while agitation in closed reactors increased the rate of denitrification.