The muskellunge, Esox masquinongy, has invaded the Saint John River of New Brunswick in the last decade. Introduced as fingerlings into a small lake in the river system in the Province of Quebec, the fish moved downstream, increasing tlle species’ range and abundance. At least 60 fish have been collected in New Brunswick since 1988, most at hydroelectric dams in tl1e upper and middle stretches of the river. A limited summer and winter fishery for Muskellunge has developed in a lake in the nortllwestern part of the province. Lengths-at-age suggest that the river fish are growing rapidly. The oldest fish was VI+. Some fish of both sexes appear to mature at age III+. The presence of young-of-the-year fish and tl1e condition of the gonads indicate that spawning has occurred and that the muskie is capable of establishing self-sustaining local populations in the river.