A model for xanthan fermentation which considers the interaction between kinetic and mixing phenomena is described. Fermentation kinetics are represented by expressions reported in the literature. The volume of well-mixed regions or ＇caverns＇ can be calculated as a function of the power number and apparent yield stress according to expressions proposed in the literature. A comprehensive data base containing aerated power data was used to predict the power drawn. The model assumes that kinetic changes occur only in well-mixed caverns or ＇actives zones＇. The model was able to estimate an agitation rate profile which allowed complete broth mixing during fermentation at a minimum power drawn. The model predictions were able to indicate that the evolution of gum concentration is a function of impeller speed (if insufficient agitation power is drawn) and this is due to a mixing problem. In model-controlled fermentations, the bacterial growth and the xanthan productivity (g/litre h(-1)) of the process were lower than those obtained in cultures in which the dissolved oxygen tension (DOT) was kept above 10% saturation (DOT-controlled), however, the specific xanthan productivity (g xanthan/g cells h(-1)) remained unchanged. DOT-controlled culture consumed at least 46% more power than the model-controlled fermentation When compared at the same xanthan gum concentration, the broths obtained with model-controlled fermentations were more viscous than those obtained with DOT-controlled cultures.