A biofilter was installed for the removal of odorous gases from the pit ventilation air of a slatted floor pig finishing barn and was studied over a 2 year period. The biofilter was effective in removing odour from the air. Design parameters for size of the filter (area and depth) are discussed along with the importance of maintaining moisture content within limits. Bacterial populations were cultured and related to ambient temperature and moisture content. Limits of moisture content were determined, but temperature had little effect on bacterial populations.