The kinetics of anaerobic digestion of cow manure has been studied using a batch bioreactor with biomass immobilized on zeolite and a control digester. The apparent kinetic constant, K(o), and the mean rate of methane production decreased considerably with substrate concentration in the control digester showing a major inhibition process due to ammoniacal nitrogen accumulation. However, in the zeolite digester, the kinetic constant was virtually invariable in the COD range studied (1.4-9.8 g l-1) due to the ionic exchange between the support and substrate. In addition, the yield coefficient was 5 times larger than the control digester for high substrate concentrations (9.8-10.3 g COD l-1).