Eight isolates of Fusarium oxysporum f.sp. vasinfeclum (FOV) were tested for levels of pathogenicity on 45-day-old greenhouse grown seedlings of six cotton genotypes. Isolates differed significantly (p=0.0000) in their pathogenicity on the genotypes. Similarly, differences among genotypes were very highly significant (p=0.0000) when they were tested against the isolates. isolate x genotype interaction was a very highly significant (p=0.0094) source of variation in wilt incidence suggesting that isolates responded differently to the different genotypes. These results imply that the pathogenicity of the tested isolates is a mixture of both aggressiveness and virulence and there are significant differences among isolates in both types of pathogenicity. Similarly, resistance of the tested genotypes is also a mixture of both horizontal and vertical resistance, and the genotypes significantly differ in both types of resistance. Assessment of the relative contribution of each of these factors in the explained (model) variation in wilt incidence revealed that isolate aggressiveness accounted for 27.79% of the explained variation, horizontal resistance of the genotypes accounted for 34.79%, and virulence of the isolates or vertical resistance of the genotypes accounted for 35.12%. Peroxidase isozymes from mycelium and conidia of the isolates were separated by polyacrylamide gel electrophoresis (PAGE). and the obtained banding patterns were visualized by using a specific staining system. Data for virulence or aggressiveness (dependant variables) and amounts of isozymes (independent variables or predictors) were entered into a computerized stepwise multiple regression analysis. Using the predictors supplied by stepwise regression, five regression models were constructed to predict virulence of isolates on genotypes. Three of the generated models proved effective in predicting isolate virulence on the genotypes 417798, worse, and 545798. Coefficient of determination (R2) values of the models were 98.19, 83.01, and 67.76%, respectively. it is noteworthy that the one- variable model of aggressiveness showed the lowest R2 value (44.25%), which may indicate that peroxidase isozymes are of limited value as biochemical markers to predict aggressiveness of F0)! isolates. The results of the present study suggest that peroxidase isozymes of isolates may provide a sUpplementary assay to greenhouse tests to quantify virulence of FOV isolates.