The effect of chitosan on the growth, morphology, and ultrastructure of Fusarium oxysporum f. sp. radicis-lycopersici was investigated.  Chitosan was found to inhibit the radial growth of F. o. radicis-lycopersici with an optimal effect at concentrations ranging from 3 to 6 mg/ml.  Light-microscope observations showed that chitosan induced morphological changes, including hyphal swelling and distortion.  Ultrastructural changes observed in chitosan-treated cells included alteration of the plasma membrane and pronounced aggregation of the cytoplasm.  An additional reaction was the abnormal deposition of an amorphous material that contained large amounts of chitin, as shown by its strong affinity to wheat germ agglutinin.  Chitosan stimulated the activities of both β-1,3-glucanase and chitosanase but did not affect chitinase and chitin deacetylase activities.  The possible modes of action of chitosan are discussed relative to morphological and ultrastructural alterations, abnormal depositions of wall-like material, and the enhanced enzymatic activities.