Quinqueserialis quinqueserialis (Trematoda: Notocotilidae) is a widespread species in the Holarctic that parasitizes the cecum of the muskrat. The paper presents for the first time the results of micromorphological studies of parenchymal organs of the muskrat at a high intensity of invasion by Q. quinqueserialis. In infected muskrats, activation of the B-cell and macrophage components of the immune system was established against the background of inhibition of granulocytic reactions in the spleen, various types of dystrophies were detected in the liver, with the formation of functional maladjustment of the organ, inflammatory changes were recorded in the kidneys with signs of alterative, necrobiotic processes. The development of destructive pathomorphological processes indicates the participation of trematodes in the pathogenesis of this parasitosis. The obtained data provide new information for understanding the functions of lymphoid and parenchymal organs in wild rodents with trematodosis.