Helminthes are a group of invertebrates living in the internal organs of a wide range of hosts. Helminthes are evolutionarily adapted to a relatively stable environment, formed by the host’s ability to maintain homeostasis under variable environmental conditions and pollution. Therefore, it is interesting to evaluate the ability of the parasite to independently adapt to adverse environmental factors. In this paper, we hypothesized that the host’s defense system response to a challenging environment is more pronounced than that of the parasite living inside the host. To study the activation of the protective mechanisms involving biotransformation of xenobiotics and antioxidant defense, the activity of the enzyme glutathione S-transferase (GST) and the concentration of reduced glutathione (GSH) were measured in an obligate parasite Triaenophorus nodulosus and its host – pike Esox lucius L. living in a pond contaminated by wastes from the Kostomuksha iron mine and ore dressing mill (Northern Karelia). Biological samples obtained from intact Lake Kamennoe were used for reference. The fish collected from the disturbed environment had an elevated level of reduced glutathione in all organs, whereas the activity of glutathione S-transferase increased only in kidneys and muscles. In T. nodulosus from the contaminated lake a decline of GST activity and the absence of response from GSH were observed. The results show that the external environment affects the intestinal parasite indirectly, via the variations of the physiological and biochemical status of the host, as a result of the host-parasite co-evolution.