Growth inhibition towards Rhizopus nigricans, Fusarium oxysporum f. sp. radicis-lycopersici, Verticillium albo-atrum and Pythium ultimum was observed in vitro using a purified chitosanase from an actinomycete, Streptomyces sp, strain N174. The corresponding gene, with its own signal peptide, was inserted into pBI121.7 shuttle vector to transform tobacco. Transgenic plants were analysed for chitosanase activity by a sodium dodecyl sulfate-polyacrylamide gel electrophoresis assay. Two major and one minor active electrophoretic forms were detected in transgenic tobacco. Some chitosanases were recovered not only in leaf homogenates but also in leaf intercellular fluid extracts. One chitosanase electrophoretic form migrated very closely to the purified Streptomyces mature protein while the others corresponded to molecules of higher molecular mass. The N-terminus sequence was determined for one of the three chitosanase forms. It exhibited a different signal peptide cleavage site when compared to the mature chitosanase from Streptomyces. This is the first report on the expression of an active chitosanase gene with antimicrobial potential in plants.