Two juvenile blueback herring (Alosa aestivalis) were caught in Lake Ontario in October 1995, the first record of this anadromous marine clupeid in the Great Lakes. Blueback herring most likely gained entry to Lake Ontario via the Erie Barge Canal, a navigation canal that links the Mohawk-Hudson rivers, which drain to the Atlantic Ocean, to Oneida Lake, which drains to Lake Ontario through the Oneida-Oswego rivers. Blueback herring ascend the Hudson River to spawn and were first reported from the upper Mohawk River in 1978. They currently spawn in several of the upper Mohawk's tributaries, including one about 430 km from the ocean but only 25 km from Oneida Lake. They were first found in Oneida Lake in 1982 and, in fall 1994, large numbers of juvenile blueback herring were found moving down the Oswego River. In the southern United States, blueback herring established self-reproducing populations in several reservoirs, and thus they have the potential to colonize Lake Ontario. If blueback herring became established in Lake Ontario, they could spread to other Great Lakes and impede recovery of depressed populations of indigenous fishes, like lake herring (Coregonus artedi) and lake trout (Salvelinus namaycush), through competition with, or predation on, their larvae.