In this study, 16S rRNA- and rDNA-based denaturing gradient gel electrophoresis (DGGE) were used to study the temporal and spatial evolution of the microbial communities in a compost biofilter removing H2S and in a control biofilter without H2S loading. During the first 81 days of the experiment, the H2S removal efficiencies always exceeded 93% at loading rates between 4.1 and 30 g m(3) h(-1). Afterwards, the H2S removal efficiency decreased to values between 44 and 71%. RNA-based DGGE analysis showed that H2S loading to the biofilter increased the stability of the active microbial community but decreased the activity-based diversity and evenness. The most intense band in both the RNA- and DNA-based DGGE patterns of the H2S-degrading biofilter represented the sulfur oxidizing bacterium Thiobacillus thioparus. This suggested that T. thioparus constituted a major part of the bacterial community and was an important primary degrader in the H2S-degrading biofilter. The decreasing H2S removal efficiencies near the end of the experiment were not accompanied by a substantial change of the DGGE patterns. Therefore, the decreased H2S removal was probably not caused by a failing microbiology but rather by a decrease of the mass transfer of substrates after agglutination of the compost particles.