The observation of the wildlife sanctuary "Gladyshevskii" (Karelian Isthmus, Leningrad oblast) revealed the presence of the fresh-water pearl mussel Margaritana mar gar it if era included into the Red Data Book of the Russian Federation in its rivers. The mollusk population is very small, and the rate of its reproduction is very low. Old-aged mollusks mainly inhabit the rivers investigated, they remained from the time of the existence of native salmon population. One of the efficient measures to restore Margaritana margaritifera is release of salmon young fishes during the spawning period of salmon in spring and/or late summer or immediately before it.