Two new constructed strains of X. campestris XLM1521 and XMT1, either alone or in mixed cultures were used for the production of xanthan gum in batch fermentations from whey. Fermentations were carried out at three different pH levels of 6.0, 7.0 and 8.0 and the results were compared with those previously reported for fermentations in Erlenmeyer flasks without controlling the pH. The kinetics of cultures of the strain X. campestris XLM1521 were studied in a batch reactor at constant pH values. A mathematical model, based on the Luedeking-Piret equation, was used and experimental design was employed in order to correlate model＇s parameters with pH variation. The highest xanthan gum concentration, 17.3 g/l, was observed at pH 8.0. This is the largest amount of xanthan gum reported so far.