The aim of the study was a pathomorphological evaluation of the effect of the new drug Clozalbent F (Clozalbent F) on populations of the trematode Fasciola gigantica (Cobbold, 1855) with monoinvasion of fasciolеsis in cows. In the experiment, on days 7-10 after a single oral application to cows spontaneously infected with trematodes Fasciola gigantica, the effects of the drug Clozalbent F (Clozalbent F) at a dose of 1.25 g/10 kg of live weight appear in the form of irreversible pathomorphological changes in organs and tissues marit and premarital. In the body of trematodes, carbohydrate, protein, water and salt metabolism is disturbed, toxins are formed, and hydropic dystrophy of tissues increases with large amounts of water in them. Hydration of F. gigantica cells and tissues causes them to swell, then decay and necrosis. Under the influence of Closalbent F, vacuoles are formed in the tegument of trematodes in the intercellular spaces filled with turbid moisture. The drug enters the body of F. gigantica through the neo-dermis, causing irreversible path changes (destruction, disorganisation, decomposition, autolysis, protein, carbohydrate, hydropic dystrophy, necrobiosis and necrosis) of cells and tissues in mature trematodes. On the 7-10th day of the experiment, after the action of the Clozalbent F at a dose of 1,25 g/10 kg of live weight, F. gigantica marita showed complete disintegration of tegument, suckers and genitalia cells, which was confirmed by histological methods (TS, BFS, SA, CHIC) and testifying to its high efficiency and biosafety fasciolеsis in cow.