The diversity and abundance of ammonia-oxidizing bacteria (AOB) in activated sludge were compared using PCR-DGGE and real-time PCR assays. Activated sludge samples were collected from five different types of wastewater treatment plants (WWTPs) mainly treating textile, paper, food and livestock wastewater or domestic sewage. The composition of total bacteria determined by PCR-DGGE was highly diverse between the samples, whereas the community of AOB was similar across all the investigated activated sludge. Total bacterial numbers and AOB numbers in the aerated mixed liquor were in the range of 1.8x10(10) to 3.8x10(12) and 1.7x10(6) to 2.7x10(10) copies/l, respectively. Activated sludge from livestock, textile, and sewage treating WWTPs contained relatively high amoA gene copies (more than 10(5) copies/l) whereas activated sludge from food and paper WWTPs revealed the low number of amoA gene (less than 10(3) copies/L). The value of the amoA gene copy effectively showed the difference in composition of bacteria in different activated sludge samples and this was better than the measurement with the AOB 16S rRNA or total 16S rRNA gene. These results suggest that the quantification of the amoA gene can help monitor AOB and ammonia oxidation in WWTPs.