Acetamide degradation was investigated in a bench-scale upflow anaerobic sludge-blanket (UASB) reactor, successively fed with acetamide, acetate and acetamide, over a period of 343 days, at different hydraulic retention times (t(HR)) The reactor was seeded with the sludge previously described [Guyot et al. (1994) Appl Microbiol Biotechnol, 42:452-456], in which methanogenesis from acetamide was performed through a synergistic relationship between an acetamide-degrading, aerobic rod and methanogens. When the reactor was fed acetamide, the chemical oxygen demand (GOD) removal efficiency was 86% at volumetric loads less than 1.18 kg COD m(-3) day(-1). At higher volumetric loads, the efficiency decreased markedly, e.g. 50.9% at a volumetric organic load of 3.39 kg COD m(-3) day(-1) (1 day t(HR)) with an accumulation of both acetamide and acetate. The same reactor, when fed with acetate at t(HR) 1 day, reached a high COD removal (99%). Evidence of the inhibition of acetate degradation by acetamide is presented. After a long period (135 days) without feeding the reactor with : acetamide, the sludge reactor was still capable of degrading acetamide when this substrate was supplied again. It seems that the synergistic degradation of acetamide by aerobes and methanogens present in the UASB reactor sludge is stable over a long period (343 days), in spite of limiting concentrations of dissolved oxygen in the feed.