Summary 1 This laboratory study examined the effect of a gradient of UV-B radiation (280–320 nm) on photosynthesis and food quality of periphyton, the trophic base of many freshwater benthic communities. Four irradiances of UV-B (0, 0.6, 1.2, and 2.3 W m-2) were delivered by UV-B lamps (313 nm peak irradiance) over a 13-day period in the first experiment and over a 4-h period in the second experiment. These irradiances were roughly equivalent to 0, 1, 2, and 4 times the ambient biologically effective (DNA) midsummer, midday UV-B irradiance in Tennessee. 2 Rates of photosynthesis and photosynthetic pigments were significantly reduced by irradiances greater than ambient during the 13-day experiment, suggesting that food supply rates to grazers would be depressed by increases in current UV-B levels. Effects on community structure were minor, but mean diatom cell size decreased at higher UV-B irradiances. 3 Irradiated periphyton was fed in surplus to juvenile snails (Physella gyrina) in the first experiment as a bioassay for food quality. Snail growth was the same on all four diets, suggesting that UV-B did not affect food quality. Nitrogen and phosphorus content of the periphyton were not affected by UV-B, either. 4 Photosynthesis by low-biomass periphyton in the second experiment was significantly depressed by irradiances above ambient after only 4 h. Photosynthesis by the high biomass periphyton was not significantly affected by UV-B, suggesting that self-shading reduced UV-B effects. [ABSTRACT FROM AUTHOR]