A method to evaluate microbial activity and growth in an eight-stage, trickling bed reactor, packed with peat particles, was developed. Steady-state measurements were made of the substrate degradation rate, based on the change in chemical oxygen demand (GOD), and the biological rate of oxygen consumption based on the difference between inlet and outlet oxygen concentrations in the gas phase. From these values, a global yield coefficient (Y-X/S) was evaluated in terms of g of total biomass produced per g of COD consumed. Using this method, Y-X/S was 0.23 +/- 0.03 from days 10 to 23. This correlated well with a Y-X/S of 0.29 g of biomass produced per g of substrate consumed as determined in the first stage of an eight-stage reactor. This method could be applied to suspension culture and fixed bed bioreactors where obtaining a homogeneous, biomass sample is difficult.