In this paper, the improvement of a fed-batch fermentation from the point of view of an industrial xylanase production process is described. The Bacillus strain chosen for this study is able to produce high quantities of a xylanase that is suitable to be used as bleach boost agent hi chlorine-free bleaching sequences of paper. pulp, ft was found that xylo-oligosaccharides (hydrolysis products from xylan by xylanase action) were indispensable for induction of the enzyme synthesis, but that their presence in quantities of only 0.1 g dm(-3) xylose equivalents led to catabolite repression. A substrate-limited fed-batch process, that is the most adapted, was furthermore Improved with regard to nutrient requirement of the microorganism, especially the nitrogen source. A process with constant supply of a culture medium containing xylan, peptone and mineral nitrogen was able to produce 20 240 nkat cm(-3) with a productivity of 910 nkat cm(-3)h(-1), which places the process among the best ever reported.