The in vitro antifungal properties of chitosan and its role in protection of soybean from a sudden death syndrome (SDS) were evaluated. Chitosan inhibited the radial and submerged growth of F solani f. sp. glycines with a marked effect at concentrations up to 1 mg/ml indicating antifungal property and at 3 mg/ml was able to delay SDS symptoms expression on soybean leaves for over three days after fungal inoculation when applied preventively. Chitosan was able to induce the level of chitinase activity in soybean resulting in the retardation of SDS development in soybean leaves. However, the SDS symptoms gradually appeared and were associated with the reduction of chitinase activity level after five days of infection period. These results suggested the role of chitosan in partially protecting soybeans from F solani f. sp. glycines infection. (c) 2006 Elsevier Ltd. All rights reserved.