Acetone butanol ethanol was produced in a continuous immobilized cell (biofilm) plug-flow reactor inoculated with Clostridium beijerinckii BA101. To achieve high reactor productivity, C. beijerinckii BA101 cells were immobilized by adsorption onto clay brick. The continuous plug-flow reactor offers high productivities owing to reduced butanol inhibition and increased cell concentration. Although high productivity was achieved, it was at the expense of low sugar utilization (30.3%). To increase sugar utilization, the reactor effluent was recycled. However, this approach is complicated by butanol toxicity. The effluent was recycled after removal of butanol by pervaporation to reduce butanol toxicity in the reactor. Recycling of butanol-free effluent resulted in a sugar utilization of 100.7% in addition to high productivity of 10.2 g/(L-h) at a dilution rate of 1.5 h(-1). A dilution rate of 2.0 h(-1) resulted in a reactor productivity of 16.2 g/(L.h) and sugar utilization of 101.4%. It is anticipated that this reactor-recovery system would be economical for butanol production when using C. beijerinckii BA101.