The effects of stepwise seeding on the start-up of an anaerobic biofilter were studied in four semi-continuous reactors; The COD removal efficiency, gas production, and biomass accumulation in the reactor increased with the number of seedings. The operational characteristics of the four types of anaerobic upflow biofilters reported in a previous paper [Y. S. Choi, E. B. Shin, H. N. Chang and H. K. Chung, Bioprocess Engng, 4, 275-281 (1989)]: non-packed (NP-AUBF), two-stage (2S-AUBF), multi-stage (MS-AUBF) and fully-packed (FP-AUBF), were further investigated at a higher loading rate (8 kg COD m(-3) day(-1)) and waste strength (6000 mg COD l(-1)). The conversion efficiencies of the 2S-, MS-, and FP-AUBFs were 70-95%, while that of the NP-AUBF was lower than 70%. The 2S-AUBF had almost the same treatment efficiency as the FP-AUBF and the problems of plugging and channeling were less than those observed in the FP-AUBF. The profiles of COD concentration and biomass buildup along the reactor length showed a sharp decrease at the entrance of the reactor and subsequent decrease in the remaining region. Copyright (C) 1996 Elsevier Science Ltd.