In this study we investigated the effects of tannic acid on fitness-related traits in gypsy moth larvae from two differently adapted populations. Thirty two full-sib families from oak (suitable host plant, Quercus population), and twenty six full-sib families from locust-tree (unsuitable host plant, Robinia population) forests were reared on artificial diets with or without a 5% tannic acid supplement. Tannic acid shortened the duration of larval development till the 4 th instar in the Robinia population, and decreased the mass of larvae from both Quercus and Robinia populations. Local adaptation was not recorded for any of the examined traits. In general, regardless of population origin, genetic variation (broad-sense heritability) was higher in larvae fed on the diet containing tannic acid than in larvae fed on the control diet. Variability of phenotypic plasticity was significant for the examined traits, except for larval duration traits in Quercus larvae, point-ing to the potential of gypsy moth larvae from both populations for the evolution of adaptive plastic responses to new environmental conditions and the presence of stressors. Genetic correlations between the environments were positive and significantly different from "one" and accordingly, do not represent constraint for the evolution of plasticity.