Penicillium janthinellum was cultivated in sugar cane bagasse hemicellulosic hydrolysate. The maximum xylanase activity obtained in shake flasks was 98 U ml(-1) at 60 min(-1) agitation rate, 30 degrees C and pH 5.5. The growth of the P.janthinellum in a 4 litre fermentor was studied to determine the influence of aeration and agitation rate on xylanase production. Three agitation rates (100, 300 and 400 min(-1)) and three aeration rates (0.2, 0.4 and 0.6 vvm) were used. Xylanase synthesis was growth associated and a function of K-L a. The highest enzymic activity (8.9 U ml(-1)) was obtained at a very low oxygen supply, at a K-L of a 1.24 h(-1).