Biodegradation of free gossypol (BFG) by Candida tropicalis ZAU-1 was carried out under solid state fermentation (SSF). The effects of medium composition and incubation conditions on the BFG were investigated and a suitable solid medium and optimum incubation conditions for fermentation process were therefore established. Maximal BFG (92.29%) occurred after 60 h of incubation at 30-35 degrees C in optimized solid medium containing cottonseed meal 60%, wheat bran 20%, rice bran 18%, rice wine spent grain (RWSG) 2%, molasses 1%, MgS04-71-120 0.5% and KH2PO4 1.5% at the optimal conditions including moisture content of solid substrate 50-55% (w/w), initial pH 4.0-6.0 and inoculum level at 1 x 10(7) cell/g of solid substrate. (c) 2006 Elsevier Ltd. All rights reserved.