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Key habitat for northern Dolly Varden (Salvelinus malma malma) is described and quantified for all known populations in Canada from existing literature and field surveys conducted in 2008 and 2009. We define key habitat as an area which, if disrupted or lost, would have serious implications for the survival of existing populations of that taxon. Adult Dolly Varden use areas associated with perennial groundwater springs for spawning and overwintering. Juveniles rear in low velocity side channels and small streams (< 10 m wide) during the summer and join adults at overwintering sites in most areas during the winter. Near shore coastal areas are important feeding grounds for anadromous populations. Winter habitat, which typically corresponds with spawning habitat, is pivotal for survival of all northern Dolly Varden populations. This habitat is comprised of small groundwater-fed pools in isolated headwater reaches, which remain open all winter, whereas most of the remainder of the river freezes to the bottom. Surface area calculations of this habitat ranged from 0.0001 km 2 to 0.1903 km 2 for the rivers that were surveyed. The total amount of known winter habitat available for northern Dolly Varden is estimated to be 0.6315 km 2 for all rivers combined. Appropriate measures should be taken to monitor and protect key winter habitat.