We investigated the communities of ammonia-oxidizing bacteria (AOB) in activated sludge collected from eight wastewater treatment systems using polymerase chain reaction (PCR) followed by terminal restriction fragment length polymorphism (T-RFLP), cloning, and sequencing of the α-subunit of the ammonia monooxygenase gene (amoA). The T-RFLP fingerprint analyses showed that different wastewater treatment systems harbored distinct AOB communities. However, there was no remarkable difference among the AOB T-RFLP profiles from different parts of the same system. The T-RFLP fingerprints showed that a full-scale wastewater treatment plant (WWTP) contained a larger number of dominant AOB species than a pilot-scale reactor. The source of influent affected the AOB community, and the WWTPs treating domestic wastewater contained a higher AOB diversity than those receiving mixed domestic and industrial wastewater. However, the AOB community structure was little affected by the treatment process in this study. Phylogenetic analysis of the cloned amoA genes clearly indicated that all the dominant AOB in the systems was closely related to Nitrosomonas spp. not to Nitrosospira spp. Members of the Nitrosomonas oligotropha and Nitrosomonas communis clusters were found in all samples, while members of Nitrosomonas europaea cluster occurred in some systems.