Continuous hydrogen production by fermentation of Avicel hydrolysate in an aqueous two-phase system, using Clostridium sp, strain no. 2, was investigated. Continuous hydrolysis of Avicel with a commercial cellulase preparation was performed in an aqueous two-phase system consisting of 10% polyethylene glycol and 5% dextran. The hydrolysate was continuously pumped at a dilution rate of 0.17 h(-1) into a 300-ml hydrogen fermenter. During an 81-h period of stationary culture, strain no, 2 consumed 0.92 mmol . h(-1) glucose and produced 4.10 mmol . h(-1) hydrogen.