The effects of nitrogen and phosphate in batch and continuous AEB fermentations were tested Both nitrogen- and phosphate-limited fermentations favored acid formation but not solvent production. A coupled two-stage continuous fermentation was performed for 30 days with a nitrogen-limited first stage fermentation for enhanced acid production. The bacteria from the acidogenic phase (first stage) fermentation were continuously pumped into a 14-l second stage fermentor with supplemental glucose and nitrogen for solvent production. The second stage fermentor had a maximum butanol productivity of 0.4 g l(-1) h(-1) (total solvent production was 0.6 g l(-1) h(-1)) at a dilution rate of 0.06 h(-1).