The fungus Gongronella butleri USDB 0201 was grown on sweet potato pieces supplied with different amounts of urea at 26 degreesC for 7 days under sterilized and humidified air supply. Crude chitosan was extracted from the fungal mycelia and treated with alpha-amylolytic enzyme to remove bound glucan. The distribution of the molecular weight of the chitosan was studied by gel exclusion chromatography. The conditions for optimal production of fungal chitosan by solid substrate (SS) fermentation have been investigated. The initial pH and the amount of urea influence the yield of fungal mycelia and chitosan. The importance of chitosan production by fungi is discussed. (C) 2003 Elsevier Ltd. All rights reserved.