The microbial population of the biofilter developed for the treatment of air contaminated by solvent vapours was evaluated using plate count techniques. The number of total heterotrophic bacteria and bacteria utilizing contaminant as the only source of carbon and energy was estimated during the exchange of the filter-bed material and start-up of its operation. Spore-forming bacteria (Bacillus spp.) occupied a significant part of the filter-bed niche. It is proposed that sporulation helps bacteria to survive during the breaks in operation of the biofilter.