The effects of culture pH ranging from pH 3.0 to 6.0 on cell growth, exopolysaccharide biosynthesis and molecular weight distribution of exopolysaccharides of Antrodia camphorata were examined both in shake flask culture and in a stirred tank fermenter. In a controlled pH stirred tank fermentation, the optimum pH for cell growth was 4.0 with a cell yield at 0.3 g/g while that for exopolysaccharide formation was 5.0 with a product yield at 5.05 mg/g. A relatively high molecular weight exopolysaccbaride with a lower yield was obtained at low pH values while a relatively low molecular weight exopolysaccharide with a high yield was obtained at higher pH values. The average molecular weight of the exopolysaccharide in the flask culture was higher than that in the stirred tank fermenter. A two stage pH process that maximized product formation was demonstrated with a high product yield of 148 mg/l with the relatively high average molecular weight of 2.18 x 10(5) (C) 2003 Elsevier Ltd. All rights reserved.