In this study, the temporal and spatial differences in the microbial community structures in a horizontal biotrickling filter (HBTF) for simultaneous biodegradation of H2S and NH3 Was investigated by denaturing gradient gel electrophoresis (DGGE) of polymerase chain reaction (PCR) amplified 16S rRNA gene fragments. Over 90% H2S and 95% NH3 were removed simultaneously under the loadings up to 137 g m(-3) h(-1) in most cases. PCR-DGGE fingerprints revealed high population diversity of the biofilm in the HBTF. The microbial community structures in the HBTF could adapt to changing operating conditions, e.g., inlet loadings and substrates starvation, while they were relatively stable under the constant loadings. In addition, the microbial Community structures differed as a function of the section of the HBTF during early operational period, while they were much more similar over extended operation. (C) 2009 Elsevier Ltd. All rights reserved.