Status of, and changes in, the bacterial communities at two acclimation stages (with- and without-ultrasound) in a small 70 degrees C ultrasound-enhanced anaerobic reactor for treating carbazole-containing wastewater reactor were analyzed by PCR-DGGE and real-time PCR techniques. PCR-DGGE results indicated that a large number of bands occurred in the whole sludge samples. Pseudomonas sp., Comamonas sp., and Diaphorobacter sp. were identified as being able to utilize carbazole as a carbon source, survive in an anaerobic and ultra-high-temperature environment and become dominant bacterial taxa during the with-ultrasound stage in the reactor. Total bacterial density in the with-ultrasonic stages was 10 x higher than in the without-ultrasonic treatment. The proportion of Pseudomonas was relatively stable at 0.13%-0.15% in both acclimation stages, which indicates that Pseudomonas can flourish and promote carbazole degradation either with or without-ultrasound. These studies provide information on carbazole degradation under ultra-high-temperature conditions in an anaerobic environment. (C) 2009 Elsevier Ltd. All rights reserved.