Cadmium ion (Cd(2+)) toxicity on anaerobic systems, used for organic matter removal, was assessed by studying its effect on kinetic parameters and metabolic changes. This fundamental study was performed in a continuous fixed bed anaerobic bioreactor that treated synthetic wastewater simulating domestic sewage. The biomass was immobilized on a fixed bed made of polyurethane foam. Under influent cadmium concentrations of 0.0, 0.4, 4.4 and 6.2 mg Cd(2+) L(-1) the organic matter removal efficiencies were 84%, 82%, 72% and 52%, respectively. At influent concentration of 6.2 mg Cd(2+) L(-1) the reactor had reached its limit for cadmium toxicity. In the removal of dissolved organic matter, the first-order apparent kinetic coefficients (k(1)(app)) were 0.84, 0.67 and 0.10 h(-1) for the operations with 0.0, 0.4 and 4.4 mg Cd(2+) L(-1), respectively. The apparent inhibition coefficient for cadmium (k(i)(app)) was 1.69 mg L(-1). Despite the toxic effects of cadmium on anaerobic organic matter removal at large Cd(2+) concentrations, the results demonstrated that the anaerobic process was suitable for cadmium concentrations below 29.8 mg Cd(2+) L(-1), considering the bioavailable fraction of adsorbed cadmium in the support when the cadmium influent concentration was 6.2 mg Cd(2+) L(-1).