The history of settlement of the Great Cormorant (Phalacrocoraxcarbo) on islands in Lake Onego is discussed. In the 20th century, irregular visits of birds of the Atlantic subspecies (Ph. carbo carbo) to the lake were observed only during autumn migrations. In the 21st century, Great Cormorants began to appear on the lake not only in autumn but also in summer. These arrivals were associated with the population growth of the continental subspecies (Ph. carbo sinensis) in the eastern part of the Gulf of Finland, from where birds started to disperse northward. In 2017, the first small colony was recorded on Gorelka Island (Shardon island group), where 3 elevated and several ground nests were spotted, along with 5 nestlings and about 50 birds swimming nearby. Surveying Gorelka Island on July 30, 2025, we found one nest with nestlings and 20 nests showing signs of occupation during that season. Over the 9 years of the colony’s existence, the birds’ activity caused the death of willow shrubs and most of herbaceous vegetation. A second colony was found on a neighboring island, 1.5 km apart. It contained three nests with nestlings and 27 nests showing signs of occupation. The average brood size was 2.5 nestlings (n = 11). Judging by the good condition of herbaceous vegetation, this colony was established only a few years earlier. The total number of young and adult birds in the two colonies was about 250 individuals. Based on the shape of the bare skin patch at the base of the bill (gular pouch angle), most birds in the colonies belonged to the continental subspecies (Ph. carbo sinensis). A specimen from the collection of the National Museum of the Republic of Karelia, collected near Petrozavodsk in August 1954, was identified by us as Ph. carbo carbo.