Chemostat culture of Xanthomonas campestris were obtained at a dilution rate of 0.05 h(-1) and the normal feed then supplemented with 0.58 and 1.74 mmol dm(-3) isobutyric acid (IBA). Data revealed that the organism responded to sublethal acid stress by overproducing xanthan. The acid additions led to transient zones in the continuous cultivation profiles. By adding feed containing 1.74 mmol dm(-3) IBA, volumetric growth rate immediately decreased from 0 059 to 0.026 g dm(-3) h(-1) whereas the specific xanthan formation rate increased from 0.23 g g(-1) biomass h(-1) to a maximum 0.65 g g(-1) biomass h(-1) (with 1 0 mmol dm(-3) IBA addition), before decreasing as the concentration of acid attained that of the feed. By monitoring the outlet CO2 in parallel with biomass and polysaccharide levels in the IBA fermentation a 10% diversion of the total carbon flux from biomass synthesis to xanthan biosynthesis was detected. A consistent pattern of variation in activity was detected in enzymes of intermediary metabolism, suggesting an action at the regulatory level. Enhanced activities of carbon catabolism and xanthan anabolic reactions (phosphomannose isomerase) were observed in the presence of the acid. Batch experiments carried out in the presence of IBA gave results which correlated with the undissociated acid form concentration. An undissociated acid fraction of 6.5 x 10(-3) mmol dm(-3) was calculated in a set of flasks under the same conditions and a statistically validated 12% increase in xanthan production was found. The maximum activation was determined to be below 1.1 x 10(-2) mmol dm(-3) when a 58% specific xanthan production rate increase occurred in parallel with a 35% decrease in biomass concentration.