Pathogenicity tests of five Fusarium oxysporum f. sp. lycopersici (FOL) isolates on five tomato cultivars revealed that the foliar yellowing and vascular browning were produced by all isolates, however, the isolates differed in their degree of virulence. Isolate No. 1 was the most aggressive one in inducing the disease in tomato plants. Tomato cultivars reacted differently to the infection by Fusarium isolates. Pritchard cv. was the most susceptible, while, Supper Marmande cv. gave the highest level of resistance. In vitro, using chitosan as  antifungal agent with concentration of 1000 ppm significantly inhibited the myclial growth of all tested Fusarium isolates. In greenhouse experiments, the treatment of  tomato seeds or transplants roots with chitosan at concentration of 500 and 1000 ppm reduced both foliar yellowing and vascular browning incidence of tomato plants growing in soil infested with FOL (Isolate No. 1).The treatment of  transplants roots by chitosan was more effective than the treatment of seeds.