The aerobic sludge granules cultivated at high organic loading rates could effectively convert 100-700 mg l(-1) nitrite to nitrogen gas with 400 or 1,200 mg l(-1) dosed acetate. The denitrifying microbial community structure of the so-cultivated granules was investigated by 16S rRNA gene sequences and localized using fluorescence in situ hybridization (FISH). The 16S rRNA gene phylotypes in the clone library and FISH probes used exhibited high diversity among the bacteria and denitrifying communities, with the members of Betaproteobacteria predominant that were closely related to families Comamonadaceae, Nitrosomonadaceae, Alcaligenaceae, and Rhodocyclaceae. The confocal laser scanning microscope and staining test revealed that active microbial community principally distributed at 200-250 mu m beneath the outer surface, embedded in extracellular polymeric substances.