Seasonal changes in the density of epilithic algae were determined from June 1975 to November 1976 at three sites on the north shore of Great Slave Lake. Densities increased rapidly in May immediately after the disappearance of ice, showed only small fluctuations during the summer, and waned during October. Microscopic algae, mainlyFragilaria capucina, Gomphonema olivaceum, G. ventricosa, Cymbella ventricosa, and other diatoms, reached peak abundance (2 × 109 μm3/cm2) on irregular occasions (July, September, October) throughout the growing season. However, filamentous algae, mainlyUlothrix zonata, reached maximum densities (60 mg/cm2dry weight) during July of both years.Additional survey collections of epilithic algae were made at 14 sites in the east arm of the lake during June and July of 1975. The most common species includedTabellaria flocculosa, Rhopalodia gibba, Cymbella angustata, C. microcephala, Anomoeoneis vitrea, Achnanthes minutissima, andUlothrix zonata. Algal densities were considerably less than those recorded from the north shore, a reflection of lower nutrient and alkalinity levels. Maximum abundances for microscopic and filamentous algae were 1.05 × 109 μm3/cm2and 4.1 mg/cm2respectively.