The article deals with the peculiarities of the ontogeny of plants of the Heracleum mantegazzianum Sommier & Levier species. The sequence of the ontogenetic cycle of plants is analyzed and characterized. The age spectra of H. mantegazzianum populations in the Kamianets region were studied. To study the ontogenetic structure of populations, 5 experimental plots of10 m2 were laid out. It was established that the ontogeny of H. mantegazzianum is represented by6 ontogenetic states: seedlings (p), juveniles (j), imatures (im), virgin (v), generative (g) and senescents (s). According to the ontogenetic structure, the cenopopulations of all plots are characterized as normal, full-membered. The maximum of the age spectrum falls on seedlings (73,7%, site 3). The age spectrum of all studied populations was determined as left-handed, probably due to the high seed productivity of the species. The average population density for each study site was determined. The analysis showed that the average density varies from 12 to 17 individuals per 1 m2. The highest number of plants was observed in plots 3 and 5, while in other plots the ratio of the number of plants is relatively the same.