 The study of the infestation of fish in the Serratif Reservoir by the Cestod trianophorous Crassus revealed that it primarily affects Gobi species, as well as other species not native to the reservoir. The Cestod reaches maturity in pike, which are abundant in the reservoir, suggesting that Gobi serve as a primary source of food for the Cestod. This suggests that the Cestod has successfully adapted to its new environment. This article was authored by OV Myneva and A.K. Meneve.