The biomass, community structure, and photosynthetic activity of epilithic periphyton of the north shore of Lake Superior were investigated during the summers of 1966, 1967, and 1968. Pigment ratios indicated predominance of Chrysophyta. The standing crop on oven-dry basis was 156 gram/square meter. Net production in 1967 averaged 1.01 gram carbon fixed per square meter/day. Daily increment of chlorophyll was 0.57 milligram/square meter. Up to 40 foot depth, periphyton can be 5 to 6 times as important in primary production as the phytoplankton. Q-sub-10 for conditioned samples at light saturation ranged from 1.24 to 2.48. The compensation point varied from 80 to 130 foot-candles. The efficiency of energy utilization by periphyton was 0.82, a value typical for algal communities. It is assumed that periphyton will provide a baseline for estimating the advance of eutrophication of Lake Superior. (WRSIC abstract)