Characteristic representatives of the fauna of lakes Ladoga and Onegо are considered: fishes, molluscs, сrustacea and oligochaeta. The pathways of their formation varied not only between the lakes, but also within them depending on the geological history and climate. In some cases, characteristics of the fauna disagree with the presumable Pleistocene or Holocene age of the geological conditions of the area. Some authors attribute the great morphological similarity of relic crustaceans and fishes of Lake Ladoga with the marine predecessors to its longer connection to the sea, but it may be due also to the twice greater mineralization of its waters as compared with Onegо. It is shown that some of the sea relicts are not in fact sea relicts.