Polymerase chain reaction analysis in combination with denaturing gradient gel electrophoresis (DGGE) was used to determine changes in the composition of the bacterial community of a bioreactor during ammonia removal. A minimum of 13 bands were observed in the DGGE profile. Phylogenetic analysis revealed that Phylum Proteobacteria was predominantly represented in the bacterial community of the bioreactor, followed by Firmicutes, Actinobacteria, and Flavobacteriaceae. However, the occurrence and predominance of specific bacterial species varied with the concentrations of NH(3) introduced into the bioreactor. The complexity of the bacterial species generally decreased with increasing inlet NH(3) concentration. Based on the characteristics of the identified species, there is a potential for nitrification, denitrification, nitrate reduction, nitrite reduction, and ammonia assimilation to occur simultaneously in the bioreactor. The strains identified in this study are potential candidate strains for the purification of waste gases containing high concentrations of NH(3). (C) 2009 Elsevier Ltd. All rights reserved.