Gas mixture of H 2S and NH 3 in this study has been the focus in the research area concerning gases generated from the animal husbandry and the anaerobic wastewater lagoons used for their treatment. A specific microflora (mixture of Thiobacillus thioparus CH11 for H 2S and Nitrosomonas europaea for NH 3) was immobilized with Ca-alginate and packed inside a glass column to decompose H 2S and NH 3. The biofilter packed with co-immobilized cells was continuously supplied with H 2S and NH 3 gas mixtures of various ratios, and the removal efficiency, removal kinetics, and pressure drop in the biofilter was monitored. The results showed that the efficiency remained above 95% regardless of the ratios of H 2S and NH 3 used. The NH 3 concentration has little effect on H 2S removal efficiency, however, both high NH 3 and H 2S concentrations significantly suppress the NH 3 removal. Through product analysis, we found that controlling the inlet ratio of the H 2S/NH 3 could prevent the biofilter from acidification, and, therefore, enhance the operational stability. Conclusions from bioaerosol analysis and pressure drop in the biofilter suggest that the immobilized cell technique creates less environmental impact and improves pure culture operational stability. The criteria for the biofilter operation to meet the current H 2S and NH 3 emission standards were also established. To reach Taiwan's current ambient air standards of H 2S and NH 3 (0.1 and 1 ppm, respectively), the maximum inlet concentrations should not exceed 58 ppm for H 2S and 164 ppm for NH 3, and the residence time be kept at 72 s.