Xanthomonas campestris strains were cultivated on a variety of solid substrates in order to evaluate their ability to produce the exopolysaccharide xanthan. The solid substrates used were agro-industry wastes or by-products, including spent malt grains, apple pomace, grape pomace, and citrus peels, which can be obtained at a very low cost. With most substrates, xanthan yields were comparable to those obtained from conventional submerged cultivation. The basic characteristics of the process were studied on solid substrates prepared by impregnating the inert support (spent malt grains) with a solution of nutrients. Fermentations were performed on the laboratory scale either under static conditions or in rotating cultures. The production conditions were optimized so as to achieve polymer yields ranging from 32.9 to 57.1 g l(-1), as referred to the liquid volume impregnating the various substrates. The products were analysed by NMR spectroscopy, revealing a composition consistent with that of commercial xanthan.