The exopolysaccharides (EPS) production in an edible mushroom Collybia maculata was substantially increased by supplementation of organic solvents into the medium. Among the organic solvents tested as stimulating agents, 0.3% (v/v) toluene gave maximum EPS production (3.81 g/l) when supplemented at the late growth phase, by which the EPS production increased by 86% in flask cultures. This result was similarly reproduced in a bioreactor, where the maximum EPS concentration indicated 3.94 g/l after 4.5 d. A prolonged culture time after supplementation of toluene (preferably 12 h in this study) resulted in additional enhancement in EPS production (max. 4.12 g/l). The compositional analysis of the EPS produced before and after toluene treatment revealed that the EPS did not contain significant amounts of other intracellular polymeric substances. The microscopic observations evidenced that a distinct morphological difference existed between the cells before and after toluene treatment. The altered outer cell structure, as observed from transmission electron microscope (TEM) appears to be the most likely explanation for the increased EPS production in toluene-treated culture. The solvent-assisted fermentation strategy presented in this study may be worth attempting with other mushroom fermentation processes for enhancing production of EPS, particularly those with industrial potential. (c) 2006 Elsevier Ltd. All rights reserved.