Industrial wastewater bioreactors are potentially important sources of novel biocatalysts. However, the microbial populations in these bioreactors are not well characterized. The microbial community in an industrial wastewater bioreactor was surveyed by extracting DNA from a sample of activated sludge, followed by PCR amplification and sequencing of cloned 16S rRNA genes. A total of 407 cloned 16S rRNA gene sequences were compared with 88 bacterial isolates cultured from the same sample of sludge using a variety of standard media. Most of the bacteria detected by the PCR-based approach were beta-subdivision Proteobacteria, whereas most of the cultured bacteria were gamma-subdivision Proteobacteria. Only a few types of bacteria were detected by both approaches. These observations indicate that multiple techniques are necessary to characterize the microbial diversity in any complex ecosystem.