The allelopatic effect of phenolic acids contents in the leaves of six cultivars of hazel (Corylus L.) on the choice of plants by Myzocallis coryli Goetze (filbert aphid), one of the most important pest of hazel in Poland and throughout the world, was identified. The cvs White Filbert, Mogulnus, and Luizen Zellernus were more resistant to the feeding of aphids in all the years than cvs Minnas, Barra, and Halls Giant. The highest content of total phenolic acids was reported in the leaves of cvs White Filbert and Luizen Zellernuss, with a low level of acceptance by aphids. These cultivars demonstrated a high concentration of gallic acid and caffeic acid. In the leaves of cvs Minnas and Halls Giant, much infested by aphids, the total content of phenolic acids was significantly lower. Moreover, gallic and caffeic acids occurred at significantly lower concentrations. The chromatographic analysis of hazel leaf extracts revealed the presence of eight phenolic acids: gallic, protocatechuic, p-hydroxybenzoic, salicylic, chlorogenic, ferrulic, caffeic, and α-resorcinolic. The leaves of the tested cultivars, irrespective of the level of resistance to filbert aphid, showed a definitely higher concentration of acids, derivatives of trans-cinnamic acid, if compared to the amount of acids - derivatives of benzoic acid.