Aphids are characterized by a specific morphology and show a high biotic potential. Most species have been described from the temperate regions of the world. The purpose of the present research was to investigate the biology, population dynamics and harmfulness of Phyllaphis fagi (L.) (Hemiptera: Aphididae: Phyllaphidinae) on Fagus sylvatica (L.) and its two varieties ‘Pendula’ and ‘Atropunicea’. Demographic parameters of this aphid were tested in field conditions. Trees as isolated elements of greenery in the urban environment were taken into the consideration. The highest density of overwintering eggs was observed in the bark crevices of forking shoots. The shortest prereproduction and reproduction periods and the highest fertility was exhibited in the second generation of aphids. Pre-reproduction and reproduction periods of successive aphids’ generations were extended while females’ fertility was reduced. Despite of varied number of aphids on F. sylvatica and its two varieties, no significant differences were stated. Feeding aphids caused leaves’ curling downwards on both sides of the mid-rib. Clear damage was visible even during the presence of few individuals on the leaf. Decorative values of trees were reduced as early as in June.