Invasive alien species have been identified as one of the major direct causes of global biodiversity loss and global ecosystem change. The problem of the invasion of alien species of mammals into ecosystems affects such aspects of biological sciences as the assessment of a possible ecological risk for a local ecosystem, various mechanisms of adaptation and evolution of invasive species in new environmental conditions. The isoenzyme spectra of lactate dehydrogenase (LDH) in the tissues of the heart, kidneys, lungs, spleen, skeletal muscle and liver were studied in three invasive species mammals: the muskrat Ondatra zibethicus (Linnaeus, 1766), the American mink Neovison vison (Schreber, 1777), and the raccoon dog Nyctereutes procyonoides (Gray, 1834), caught in the north of the European part of Russia. Muskrat and American mink are both known to be native to North America; raccoon dog, introduced from the Far East, successfully acclimatized in Karelia. It was found that the content of anaerobic LDH fractions in the lungs, spleen, skeletal muscle, and liver of the American mink and muskrat, animal species adapted to the aquatic environment, was significantly higher compared to the terrestrial raccoon dog. The studied species of mammals adapted to the new abiotic environmental conditions of the northern region in a relatively short period of introduction, both due to their physiological and biochemical characteristics. The use of different sets of isoenzymes is one of the strategies of biochemical adaptations of animals and contributes to reveal of the mechanisms of adaptation of invasive species at the present stage of biological evolution, when the role of anthropogenic factors increases significantly.