The experiment was carried out in two SBR reactors differing in ammonia load (reactor RI--ammonia load of 130 mg N-NH4 x d(-1), reactor R2--ammonia load of 250 mg N-NH4 x d(-1)). Feeding conditions in the reactors were switched from favoring autotrophic nitrification through favoring heterotrophic processes and back to autotrophic conditions. Observations of ammonia-oxidizing bacteria (AOB) community changes were based on PCR-RFLP analysis of amplified amoA gene fragments and AOB genetic diversity was evaluated on the base of the number of different amoA gene forms. When only carbonates were introduced with wastewater restriction patterns established about day 23 and 28 at ammonia load of 250 and 130 mg N-NH4 x d(-1), respectively. In both reactors statistically higher number of different amoA gene forms was observed when only carbonates were present in wastewater in comparison to conditions in which sodium acetate was introduced to the reactors. The AOB participation in activated sludge was higher at ammonia load of 250 mg N-NH4 x d(-1) but their genetic diversity was lower in comparison with this observed at ammonia load of 130 mg N-NH4 x d(-1).