The activities of two families of nitrifying bacteria were measured by applied respirometer and fluorescence in situ hybridization (FISH) for the purpose of monitoring nitrogen removal. Oxygen uptake rates (OUR) for Nitrosomonas and Nitrobacter were separately measured with the dosing of selective inhibitors during respirometric monitoring. The FISH method was applied to analyze microorganism communities. In 5 mg NH4+-N/L solution, the total nitrogen oxygen demand (NOD) was found to be 6,672 mg DO/L-day with ammonia and nitrite oxidation rate of 4,512 and 2,160 mg DO/L-day, respectively. The oxygen consumption by endogenous oxidation was 288 mg DO/L-day and the biological oxygen demand 936 mg DO/L-day. Respirometric measurements were well matched by the FISH result, which described the presence and quantity of each nitrifying bacteria as nitrification proceeded. It was found that when coupled with OUR data, FISH could provide valuable information about the nitrogen removal process in many practical wastewater treatment systems.