Methane yield (Y-CH4) is the result of the balance between the flows of organic carbon to catabolism and anabolism in methanogenic ecosystems. In anaerobic biofilm processes, it can be easily obtained to be used to characterize the behaviour of the biofilm in important transient phases. Thus, at the beginning of the start-up period, Y-CH4 is very low, indicating an important anabolic activity of the microorganisms to build the biofilm. Then, its value increases upto a stable level corresponding to the end of the biofilm formation. The time course of Y-CH4 describes the three phases of biofilm formation with the induction, the growth and the steady state. Therefore, it is possible to compare different start-up strategies and define the best operating conditions to grow the methanogenic biofilm. In steady state operations, Y-CH4 expresses the balance of the dynamic system between the net growth of the fixed cells and the biomass detachment, depending on the shear stress and on the capacity of the solid support to retain biomass. It also reflects the detachment rate evolution in anaerobic fixed film processes. © 2005 Elsevier Ltd. All rights reserved