A floristic survey of a polje lakebed, which is full in the spring but drains completely as the season progresses, revealed a community of small mud-bottom plants including the globally rare Coleanthus subtilis, not previously recorded from Northwest Territories, Canada. The newly discovered site is 1077 miles NNW of its nearest North American location in southern British Columbia, which is the northern edge of a region of occurrence including southern British Columbia and the Columbia River in Washington and Oregon. The lakebed flora included four zones of vascular plants. Coleanthus subtilis was confined to the lowest level mud-bottom community, which had been exposed for the shortest period and included a rich assemblage of annuals including Juncus bufonius, Limosella aquatica, and Ranunculus hyperboreus. The lakebed flora included mostly widespread boreal species, and C. subtilis may be included in this category. It appears restricted to specific montane and boreal areas by its requirement of pronounced seasonal inundation in a cool climate. Coleanthus subtilis may have persisted at the Washington and British Columbia locations, near the limit of the continuous montane glaciation, since early postglacial times. The occurrence in Northwest Territories may be either a result of dispersal from unglaciated areas of Beringia nearby to the west or from the south. Although it has sometimes been considered introduced in the United States parts of its North American range, it is here considered native at all of its North American sites on the basis of: (1) its restricted and unusual habitat; (2) global rarity; (3) suffusive rarity, which is known to lead to mistaken assumptions of introduction; (4) occurrence in botanically rich regions and close association with rare native species; (5) relatively early year of collection; (6) distribution corresponding to a well recognized native pattern; (7) lack of evidence of spread to anthropogenic habitats; and (8) the fact that it is easily overlooked by early collectors as a result of only appearing at intervals of several years when water levels have dropped sufficiently.