The accumulation of both forms of butyric acid (HBu) has been used as an indicator of digester failure. However; the present experimental results indicated that the normal- and iso- HBu-degradation activities of the methanogens increase by 14-25% and 17-43%, respectively, for each gram of the biomass coming into contact with 0.02-3.89 mg of Mn, Zn, Ni, Fe and Cu. These results imply that in the presence of these heavy metals care should be taken in using HBu accumulation as an indicator of digester failure.