Extracellular polysaccharide production by Xanthomonas campestris was studied under sublethal acid stress. Chemostat cultures were obtained at a dilution rate of 0.05 h(-1) and the normal feed was then supplemented with 2.5 mM acetic acid, 1.96 mM pentanoic acid or 1.39 mM octanoic acid. Undissociated acid fractions led to an appreciable loss of biomass formation expressed by the drop in the biomass. X. campestris responded to sublethal acid stress by overproducing xanthan, and an increase in the specific xanthan production and xanthan yield was observed in all cases. The corresponding specific glucose consumption rate increased by a maximum of 45.2% with pentanoic acid. By adding feed containing 2.5 mM acetic acid, the culture biomass immediately started to decrease, falling from 0.145 to 0.103 g litre(-1) h(-1), whereas the specific xanthan production rate was stimulated from 0.282 g g(-1) cells h(-1) to a maximum 0.325 g g(-1) cells h(-1) Similar behaviour was observed in the pentanoic acid and octanoic acid fermentations. Enhanced activities of the pentose phosphate cycle and xanthan anabolic reactions were observed after acid additions.