Ammonia concentrations of 5 g N/litre or more inhibited thermophilic anaerobic digestion of cattle manure in upflow anaerobic sludge blanket (UASB) reactors. A stable digestion of cattle manure could be maintained with ammonia concentrations up to 7 g N/litre after 6 months of operation. However; the methane yield was reduced and the concentration of volatile fatty acids increased from 1 to 3 g/litre as acetic acid, compared to controls with an ammonia concentration of 3 g N/litre. The temporary strong inhibition following a one-step increase in ammonia concentration was reduced by applying a gradual increase. The specific methanogenic activity of ammonia-inhibited reactors (7 g N/litre) with acetate or hydrogen as substrate was reduced by 72 and 56%, respectively. Tests of ammonia toxicity on the acetate- and hydrogen-utilizing populations showed a higher sensitivity of the aceticlastic compared to the hydrogenotrophic methanogens; the specific growth rate for the aceticlastic methanogens was halved at ammonia concentrations of 4 g N/litre, compared to 7.5 g N/litre for the hydrogenotrophic methanogens.