Xanthan gum production by Xanthomonas campestris ATCC 1395 using sugar beet molasses as carbon source was studied. The pre-treatment of sugar beet molasses and the supplementation of the medium were investigated in order to improve xanthan gum production. Addition of K2HPO4 to the medium had a significant positive effect on both xanthan gum and biomass production. The medium was subsequently optimized with regard to molasses, K2HPO4 concentration and initial pH. Maximum xanthan gum production (53 g/l) was observed after 24 h at 175 g/l molasses, 4 g/l K2HPO4 and at neutral initial pH. Results indicate that K2HPO4 serves as a buffering agent as well as a nutrient for the growth of X. campestris. Sugar beet molasses appears to be a suitable industrial substrate for xanthan gum fermentations. (C) 2003 Elsevier Science Ltd. All rights reserved.