Aims: To investigate whether the ammonia-oxidizing bacterial (AOB) communities of replicate nitrifying bioreactors (i) co-evolve or diverge over time and (ii) are stable or dynamic during periods of complete nitrification. Methods and Results: Three sequential batch reactors (SBR) were inoculated with sludge from a municipal wastewater treatment plant, fed with ammonium- enriched tap water and operated in parallel for 134 days. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) demonstrated co-evolvement of the AOB communities over time. During start-up, temporary decreases in nitrification were noticed, and the AOB community rate of change values (Delta(t(week))) were medium to high (12-22%). During the adjacent period of complete nitrification, low AOB community dynamics were observed (Delta(t)(week) < 5%). Further pragmatic processing of the DGGE profiles revealed a high range-weighted richness and a medium functional organization of the AOB communities. Conclusions: After a start-up period, high functional stability and low dynamics of the AOB communities were observed. Deterministic rather than stochastic driving forces led to AOB community co-evolvement in the replicate SBR. Significance and Impact of the Study: Replicates in identical set-ups are reproducible, and pragmatic processing of DGGE patterns is a straightforward tool to score and compare the functionality of the bacterial communities.