The data on the influence of cestode infection on the activity of digestive enzymes in their host fish are presented. The existence of proteinase and glycosidase activity gradients along fish intestines has been confirmed. Proteinases of hosts as compared to glycosidases were shown to be more responsive of cestode infection. The infection reduced the proteolytic activity in bream and burbot and increased it in pike. Both absolute and relative activity levels of the investigated enzymes changed. In beam, the infection provoked a redistribution of the relative content of various proteinase subclasses, especially in the intestinal segments with the maximal abundance of worms. In the infected burbot, the activity of proteolytic enzymes decreased to a greater extent in the places where worms are attached, i.e., pyloric caeca. A decrease in the activity of digestive enzymes of the hosts was observed even at low intensity of invasion.