The initial rate of acetoclastic methanogenic biofilm formation was significantly higher than rates that have been reported for mixed anaerobic cultures. Biofilm development rate increased with an increase in biomass concentration and specific activity (mg substrate‐mg biomass−1‐day−1) of the mixed liquor biomass. Biofilm concentration also increased with higher specific activity. Results indicate the involvement of polysacchar‐ides in acetoclastic biofilm formation. The existence of a loosely held biofilm at flow velocities of 005 to 1–5 cm‐s−1was observed. Flow velocity increases in that range resulted in higher attached film accumulation rates. Silicone tubing was superior to Tygon tubing in facilitating biofilm formation.