Periphyton in the littoral zone of Pend Oreille Lake, a large (383 km2, deep (Zmax— 380 m)) meso-oligotrophic lake in northern Idaho, were studied to compare with pelagic productivity and to identify areas of inshore degraded water quality. Periphyton growth was determined on artificial tiles and natural substrates at 0.5 and 1.5 m depths. Means for all measures of production on natural substrates were higher than means for artificial substrate. Indicators of biomass were higher on 0.5 m tiles than those at 1.5 m; exposed sites, however, showed less difference between depths than did the more sheltered sites. Because of inorganic interference, ash-free dry weight provided a better measure of periphyton biomass than dry weight. Significant differences existed between sites, indicating greater algal growths in developed and relatively confined bays.Cymbellawas dominant on natural substrate, whileCymbella,Mougeotia, andRhizocloniumwere co-dominant on artificial substrate. Blue-green algae biomass comprised only a small part of the total biomass on either artificial or natural substrates. Periphyton biomass in Pend Oreille Lake was comparable to other lakes showing accelerated eutrophication. Despite Pend Oreille Lake's great depth and large profundal area, littoral productivity was similar to lakes more shallow in nature with higher overall productivity. In-shore water quality degradation was documented at a time when open-lake conditions indicated the lake was meso-oligotrophic. Given that changes in lake trophic status would first show up in littoral areas–compared to the relatively slow changes taking place in the phytoplankton of pelagic areas–periphyton monitoring should provide valuable refinement to in-depth limnological studies on open waters of lakes.