Xanthan gum production under several operational conditions has been studied. Temperature, initial nitrogen concentration and oxygen mass transfer rate have been changed and average molecular weight, pyruvilation and acetylation degree of xanthan produced have been measured in order to know the influence of these variables on the synthesised xanthan molecular structure. Also, xanthan gum solution viscosity has been measured, and rheological properties of the solutions have been related to molecular structure and operational conditions. The Casson model has been employed to describe the rheological behaviour. The parameter values of the Casson model, tau(o) and K-c, have been obtained for each polysaccharide synthesised under different operational conditions. Both pyruvilution and acetylation degrees and average molecular weight of xanthan increase with fermentation time Lit any operating conditions. Xanthan molecules with the highest average molecular weight have bet Il obtained at 25 degrees C. Nevertheless, at this temperature acetate and pyruvate radical concentration are lowest, Nitrogen concentration in broth does not show any clear influence over xanthan average molecular weight, although with high nitrogen source concentration xanthan with low pyruvilation degree is produced. (C) 2000 Elsevier Science Inc. All rights reserved.