The localization of various phenolic compounds in tobacco leaves and their influence on the degree of infestation by aphids ( Myzus nicotianae Blackman) were assessed. The leaves of both immune and resistant tobacco lines from the interspecific hybridization of Nicotiana gossei and N. tabacum were investigated cytochemically. A susceptible tobacco : cv "Rila 82 ", was used as a control. The presence of phenolics, flavonoids, flavonols, consed tannins and polyphenols was determined on unfixed frozen sections of leaves and fresh epidermal strips. The heads of glandular trichomes of the three tobaccos had positive cytochemical reactions for both phenolic compounds and flavonoids and negative reactions for condensed tannins. However, the heads of glandular trichomes from immune and resistant tobacco lines had more intense cytochemical reactions for flavonols than trichomes of the susceptible control. The resistance of both tobacco lines correlated with the presence of some polyphenols in glandular trichomes and leaf epidermis over vascular bundles which were lacking in the susceptible commercial tobacco cultivar. The immune, resistant and susceptible tobaccos had similar activities and localizations of peroxidase which usually takes part in the plant defense system. These features are discussed from the view-point of the role, of phenolic compounds in plant resistance to insect pests.