The bacterial community of a bulking sludge from a municipal wastewater treatment plant with anoxic-anaerobic-oxic process was investigated by combination of cultivation and 16S rRNA gene clone library analysis for understanding the causes of bulking. A total of 28 species were obtained from 63 isolates collected from six culture media. The most cultivable species belonged to gamma-Proteobacteria including Klebsiella sp., Pseudomonas sp., Aeromonas sp. and Acinetobacter sp. Further analysis of these strains by repetitive sequence based on polymerase chain reaction (rep-PCR) technology showed that rep-PCR yielded discriminatory banding patterns within the same genus using REP and BOX primer sets. While the culture-independent assessment revealed that beta-Proteobacteria was the dominant group in the bulking sample. Sequence analysis revealed that the highest proportion (14.7%) of operational taxonomic units was 98% similar to Candidatus Accumulibacter phosphatis, which is used to remove phosphorous from wastewater. Our results indicated that combining different approaches can produce complementary information, thus generate a more accurate view of microbial community in bulking sludge.