The accumulation of biofilm by Acetobacterium sp. during continuous culture in an upflow anaerobic filter (UAF) growing on methanol-formate was the result of space velocity and inlet concentrations of substrate and Co+2. To achieve good development of biofilm, a space velocity of 0.38 h(-1), inlet substrate concentrations of 125 mM of both methanol and formate, and Co+2 at 0.16 mM were required. Cell productivities in the effluent of the UAF-reactor were about 6-fold higher than in chemostat cultures (0.20 g l(-1) h(-1) for UAF and 0.035 g l(-1) h(-1) for chemostat) (previous studies), and the maximum vitamin B-12 specific concentration was 5.1 mg g cell(-1).