Technical Abstract: Specific chitin-binding isozymes of peroxidase (POX) play an important role in pathogenesis of plant diseases caused with fungi. We studied the dynamics of peroxidase activity in two varieties of cotton (Gossypium hirsutum L.); one was a susceptible and the other resistant to the plant pathogen Verticillium dahliae. After infection with a strongly and a weakly virulent isolate of V. dahliae, we observed a correlation between the level of seedling tissue lesion and peroxidase activity. Thus, the first POX activity was observed in all infected plants 2 hours after inoculation, but POX activity of the resistant variety rapidly increased and maximized 3 days after infection, while POX activity in the susceptible variety showed a slow increase that continued to increase during the remaining 8 days of experimental observation. The increase of POX activity in the susceptible variety after infection may be explained by progressive fungal colonization of cotton tissues leading to irreversible senescence. Microscopic examination of plant tissues supports this hypothesis. The more virulent isolate caused more necrosis and significantly more POX activity than the mildly virulent in both susceptible and resistant plants. Control plants showed no changes in POX activity; however, the POX activity in the control resistant varieties was higher than the control susceptible varieties. These findings indicate the potential utilization of chitin binding POX as a biochemical tool to guide breeding programs to increase resistance to V. dahliae.