In order to make clear the microbiological characteristics of the fluidized-pellet-bed bioreactor (FPB) which is a newly developed wastewater treatment device to perform coagulation, particle pelletization, biological degradation and solid-liquid separation in a single unit, the method of denaturing gradient gel electrophoresis (DGGE) was applied in this study paying attention to the microbial diversity of the granular sludge. Spread plate method was also used for enumeration of aerobic bacteria in unit weight of granular sludge. As a result, slight difference was found between the total aerobic bacteria at the bottom. middle, and top sections though the dissolved oxygen (DO) concentration decreased from about 3.5 mg/L at the bottom inlet to 0.23 mg/L at the top of the FPB bioreactor. From the DGGE finger printing, 17 common species were identified from all these sections. and certain specific species were also identified from each section. The comparability of the microbial communities in the three sections was 83.1%, indicating a very stable structure of the microbial communities. The 16 S rRNA sequence analysis results revealed that the 18 operational taxonomic units (OTUs) obtained all belong to Eubacteria. Among them 11 are Proteobacteria, 3 are Actinobacteria, 2 are low G + C gram-positive bacteria and the remaining 2 belong to other bacteria branches. The dominant microbial communities are typical aerobes or facultative anaerobes commonly encountered in conventional activated sludge. (C) 2009 Elsevier B.V. All rights reserved.