Our study aimed to evaluate the effectiveness of the enzyme defense system of the assimilation apparatus of Robinia pseudoacacia L. (black locust) against damage caused by invasive phytophages. The study focused on trees in plantations clustered in 3 age groups: I, II, and III corresponding to the age ranges of 5-10, 15-25, and 50-70 yrs. The study discovered an increase in peroxidase activity with the progression of tree age. Damaged leaves of plants from group III showed significantly greater benzidine- and guaiacol-peroxidase activity (by 68.6 and 180%, respectively) compared to undamaged leaves of the same group. Guaiacol-peroxidase activity was found to have significantly increased in trees attacked by Parectopa robiniella in groups I and III (by 15.5 and 180%, respectively), but only increased enzyme activity was recorded in trees of group II. Levels of peroxidase activity in leaves undamaged by insects (intact) were similar in young trees (groups I and II) compared to trees in group III in plantations which initially had significantly higher activity. This was confirmed through study of the isoenzyme composition of peroxidase in which leaves of plants in group ІІІ showed more activity of molecular forms of the enzyme. In the analyzed age groups of trees affected by P. robiniella, catalase activity did not significantly differ compared to that of undamaged samples. In conclusion, a peroxidase system can be considered the main defense system of black locust.