Activated sludge is widely used to treat industrial wastewater, but its efficiency is affected by a variety of factors, including toxic substances such as tetrahydrofuran (THF). In this study, we examined the toxicity of THF at different concentrations (0-320 mM) on the microbial community in activated sludge. A remarkable dose-dependent decrease in the total organic compound removal rate and culturable bacteria and fungi was observed. At THF concentrations higher than 160 mM, a decrease in pH to 3.0 was observed. The activities of five enzymes (catalase, dehydrogenase, urease, phosphatase and protease) analyzed were all significantly inhibited (p < 0.01) at THF concentrations higher than 160 mM, especially dehydrogenase activity, which lost 95.4% of its activity at 320 mM THF. Microbial community analysis by PCR-DGGE revealed a substantial shift in the community structure and a reduction in diversity at a low THF concentration (20 mM). These results suggest that THF is much more toxic than reported in the literature, indicating its acute toxicity to microorganisms. (C) 2010 Elsevier Ltd. All rights reserved.