The composition of the fish population in Lake Tolpanjarvi, located near the Russian-Finnish border, was studied for the first time. The lake was shown to be inhabited by seven fish species. Perch, roach and whitefish are the most abundant. The lake’s fish fauna comprises two forms of whitefish Coregonus lavaretus, which differ in both the number of gill rakers (18–24 in sparsely-rakered and 28–36 medium-rakered fish) and biological indices such as length-weight growth, maturation, fecundity and feeding. The presence of whitefish intraspecific forms was shown to contribute to a more complete use of the lake’s food resource. Experimental catches were dominated by older fish (7+ to 15+). This indicates a small fishing load in the lake because the lake is far away from industrial provinces. The lake has remained in a natural condition for a long time, which is uncommon for the region as well as for Russia in general. Stable and intact ecosystems that display high diversity were shown to provide a basis for the preservation of the gene pool and for assessment of the condition of aquatic ecosystems heavily affected by human activities.