<p>The big and little Baikal oilfishes are endemics of Lake Baikal, being the most numerous fish species there. These two species alone make up the group of pelagic fish of Lake Baikal. The Baikal oilfish are characterized by a dispersed vertical distribution, from the surface of the water to a depth of 1000 m or even deeper. This study provides a description of the current situation regarding parasite infection in the big and little Baikal oilfishes and, for the first time, provides an analysis of changes in the fauna and numbers of parasites in the age series of these endemics of Lake Baikal. There are 14 and 15 species in the parasite fauna of the big and little Baikal oilfishes, respectively. In total, the parasite fauna of both host species is represented by 17 species, of which 12 species are shared. Despite high degree of similarity in the species composition of parasites in the Baikal oilfish, trematodes have been found only in the little Baikal oilfish, which indicates differences in ecology between the two host species. Additionally, the studied oilfish species differ in the dynamics of parasite species richness in the fish age series: while it increases in the little Baikal oilfish, in the big Baikal oilfish, the species richness of parasites first increases and then decreases in older age groups. This discrepancy between the two Baikal oilfish species is associated with differences in the composition of the food bolus in the middle and older age groups of these fishes.</p>