We screened about a hundred microorganisms (including unidentified yeasts, fungi, and bacteria) for their ability to produce xylanolytic enzymes. About 40 of them were hemicellulolytic; among these, we selected Penicillium canescens 10-10c for detailed study because of its ability to produce an interesting enzymatic complex in quantity. The xylanase complex was cellulase-free, and had an optimal activity at pH 4.6-5.0 and 55-60 degrees C on birchwood xylan. The best production was on soya meal and wheat straw; expression of the xylanase was repressed by glucose, xylose, and lactose. The optimization of culture medium and mode (fed-batch) enabled us to improve the production three to four times. The importance of the mixing conditions on the biomass and xylanase production is also reported.??