A kinetic study of the methanogenic phase was carried out on a pilot lab scale anaerobic fluidized bed reactor (AFBR) in batch mode. An examination of the effect of initial acetate concentration, bed expansion and bed segregation is presented. Experimental data observed for the acetate removal against time were adjusted to a zero-order kinetic equation, over the chemical oxygen demand (COD) range studied (1430-5340 mg litre(-1)), independently of the bed expansion (11-37%). The kinetic constant was calculated using robust regression analysis. The zero-order kinetic constant, K-0 was between 1180-1380 mg COD litre(-1) h(-1) on the fixed bed volume basis, and the maximum specific substrate utilization rate, k, was between 145-198 mg COD g VS-1 h(-1). The kinetic behaviour was found to be different throughout the reactor, on the fixed bed volume basis and the activity at the bottom of the bed was lower than the activity in the upper region. However, on an attached volatile solids basis, the activity at the bottom level was the greatest.