Abstract: The changes in peroxidase isozymes of different varieties of Cotton seedlings after inoculation by Fusarium oxysporum f. sp. vasinfectum was studied with polyacrylamide gel electrophoresis. After inoculation, the activities of peroxidase isozymes in cotyledons were intensified and new bands appeared. The enzyme activities of susceptible varities (4 varieties) were higher than those of the resistant ones (8 varieties). The susceptible varieties invariably had one or two more isozyme bands than the resistant ones. The changes in intensity of the peroxidase isozymes of cotton seedlings after infection is coincident with the appearance of morphological symptom of the disease. The number of isozymes increased with the severity of the disease, so that the changes in the number of isozymes in the infected plant can be taken as the intrinsic "bioche- mical symptom" of the plant. There were distinct differences among the three species of cotton plants in their peroxidase isozymes. The number of isozymes is greatest in Island cotton, which is highly susceptible to fusarial wilt disease, followed by the susceptible varieties of Continent cotton and its resistant varieties, while the least is the Chinese cotton which is immune to the disease. It is suggested that the appearauce of more new isozymes in the susceptible varieties upon inoculation may be used as a criterion for screening the resistant varieties of cotton plants from those susceptible to fusarial wilt disease.