The helminth fauna of the red deer (Cervus elaphus) of the European subspecies and the sika deer (C. nippon) inhabiting the territory of the Curonian Spit National Park, “Podvorie na Khutorskoy” LLC, “Mushkino” farm and “Olen” LLC of the Kaliningrad region was revised during the period from 2017 to 2020 using methods of qualitative and quantitative intravital and postmortem helminthoscopy. The discovered helminth fauna of red and sika deer showed high species similarity, which we explained by the common range and food resources, as well as the anatomical similarity of these animals. 43 types of helminths have been identified. Of which, there were 6 species of trematodes, 4 - of cestodes and 33 - of nematodes (that is 28 were species of geo- and 15 - of biohelminths). 32 species of helminths were localized in the gastrointestinal tract of deer, 5 - in the liver, 2 - in the lungs, 2 - in the body cavities, 1 - in the muscles and1 - in the ligaments. No new and atypical species of parasitic worms have been found. The greatest threat to deer represent trema-todes of the species like Fasciola hepatica, Liorchis scotiae, Dicrocoelium lanceatum, Parafasciolopsis fasciolaemorpha, Fascioloides magna, Paramphistomum ichikawai, cestodes - Moniezia benedeni, M. expansa, Cysticercus tenuicollis, Coenurus cerebralis, Echinococcus granulosus larvae nematodes - Chabertia ovina, Oesophagostomum cervi, Trichostrongylus capricola, Tr. colubriformis, Tr. axei, Ostertagia circumcincta, O. trifurcata, Nematodirus helvetianus, Haemonchus contortus, Dictiocaulus eckerti, D. viviparous. As a result of a complex antiparasitic treatment and prophylactic measures in the “Mushkino” farm, it was possible to preserve the livestock, to stop the mortality of young reindeer, to increase the antler and meat productivity by 40%, and to increase the percentage of fawns (by 100 females) by 30%.