A kinetic study was carried out in a saline medium to assess the effect of O-2 on the two-step nitrification process: for the first nitritation step, 2-26 mg dissolved O-2 (DO) l(-1) was used and for the second nitratation step, 0.5-24 mg DO l(-1) was used. Nitritation rate was measured in the presence of sodium azide so as to inhibit nitratation. Ammonia-oxidizing (AOB) and nitrite-oxidizing (NOB) bacterial in the mixed culture were determined by 16 S rRNA hybridization. The affinity constants for oxygen O-2 of the AOB and the NOB were 1.66 mg O-2 l(-1) and 3 mg O-2 l(-1) respectively. The larger than the previously reported values of these constants might be due to the high salt content in the medium. High O-2 concentrations did inhibit the nitrification rate.