Aims: To characterize the bacterial community dynamics over 1 year in two full-scale wastewater treatment systems operated under constant conditions and exhibiting stable performance. Methods and Results: Functional stability was defined and quantified by the effluent concentration of biological oxygen demand, total nitrogen and ammonia. Community dynamics were investigated using specific PCR followed by terminal restriction fragment length polymorphism (T-RFLP) of the 16S rRNA gene. The T-RFLP results indicated that during the period of functional stability, the bacterial community structures in two full-scale wastewater treatment systems were not stable, and the average change rates every 15 days of the two systems were 22 center dot 6 +/- 6 center dot 9 and 21 center dot 6 +/- 7 center dot 3%, respectively. The corresponding species with dominant T-RFs were determined by clonal sequencing and T-RFLP. Based on Pareto-Lorenz distribution curves, it was observed that only a small number of micro-organisms were numerically dominant in the two systems. Conclusions: The results of this study showed that, throughout the period of the study, the bacterial community structure changed significantly in two full-scale wastewater treatment systems despite the stable function. Significance and Impact of the Study: The findings enrich the theory involving the relation between bacterial community dynamics and functional stability in full-scale wastewater treatment plants.