In this study, the inhibitory effects and anaerobic biotransformation of acrylic acid in computer-controlled pH-stat completely stirred tank reactors (CSTRs) with two different cultures, namely unacclimated and acrylate-acclimated acetate-enriched Methanosarcina and homogenized (crushed) granular cultures, were investigated. The microbial acclimation, influent concentration, and loading rate of acrylic acid were studied in the experiments. The experimental results revealed that methanogenic cultures at a concentration of 3200 +/- 80 mg/L as volatile suspended solids (VSS) could be acclimated to acrylic acid up to a loading rate of 220 mg/L per day (0.068 g acrylic acid/g VSS per day) in the presence of a constant acetate concentration of 2000 +/- 200 mg/L as the primary substrate after 300 days of acclimation. The same cultures (680 +/- 80 mg/L as VSS), after 80 days of acclimation to acrylic acid as the sole carbon source, transformed acrylic acid up to the loading rate of about 200 mg/L per day (0.29 g acrylic acid/g VSS per day) almost completely (>99%) to acetic and propionic acid, but could not effectively metabolize these intermediate products. Acrylate-acclimated homogenized granular cultures (6900 +/- 80 mg/L as VSS) effectively metabolized 2200 mg/L per day (0.32 g acrylic acid/g VSS per day) of acrylic acid, as the sole carbon source, after 50 days of severe inhibition. (C) 1999 John Wiley & Sons, Inc. Biotechnol Bioeng 62: 200-207, 1999.