We describe Cladophora gyrfalconium, a previously unknown alga that forms an epilithic mat, 0.04 g average ash free dry weight (AFDW)/m2), in the littoral areas of Gyrfalcon Lake. This slow growing alga is the dominant autotroph in this cold (< 70 ice-free days/yr), ultra-oligotrophic lake (average nutrients: 3 μg/L total phosphorus; 75 μg/L Total Nitrogen). No colonization by the alga was observed when an array of experimental substrata were left in the lake, and regrowth of cropped mats was negligible 2 years later. Chironomids of the genus Pseudodiamesa appear to be the only major benthic grazers in the lake's simple food web. The algal mat seems to be in a climax successional state and is thought to be a relict population.