Abstract <p>The composition of infracommunities and quantitative characteristics of infestation of underyearlings of northern pike <i>Esox lucius</i> with macroparasites have been studied in the small Ild River (a second-order tributary of the Rybinsk Reservoir) from different reaches of the river continuum (from the upper reaches to the mouth). The number of parasite species, mean abundance, and prevalence were significantly higher in the lower reaches than in the upper reaches of the river. Pike samples from different reaches significantly differed in the infection variability between the individuals. The highest coefficient of variation in the number of parasites in one fish individual was observed in the sample from the upper reaches (162%), while it was significantly lower in individuals from the middle and lower reaches (89 and 57%, respectively). Among various parasites from the lower part of the river, we have recorded manipulators of host behavior, which increase the fish availability for predators. These include several trematode species (Trematoda) of the family Diplostomidae. Underyearlings from the sample from the upper reaches were most variable in size, dominated by small individuals. The specimens from the lower reaches were characterized by a minimum variability in body length. The largest (on average) individuals were recorded in the sample from the middle reaches. Parasites are presumably a significant factor influencing the pike population structure and migratory activity, which is usually considered a sedentary species.</p>