Abstract <p>The variability and phenotypic diversity of <i>Diplostomum petromyzifluviatilis</i> metacercariae from two lamprey species (river lamprey <i>Lampetra fluviatilis</i> (Baltic Sea basin) and Arctic lamprey <i>Lethenteron camtschaticum</i> (Arctic Ocean basin)) were studied. It was established that the gostal forms of metacercariae are similar by the number of variations in the body shape, oral and ventral suckers, pharynx, as well as by their frequencies. Differences in population diversity by variations in the shape of Holdfast organ were detected. It was demonstrated that the hostal forms are similar in the range of variability of plastic traits, but differ in the distribution of frequency classes of the variation curve. It was hypothesized that a change in the distribution and nature of variation in the size of metacercariae from the Arctic lamprey is one of the population mechanisms of species adaptation to habitation at the northern border of its range.</p>