A major threat to stream ecosystems in the Appalachian region is acid-mine drainage (AMD).  The effect of three AMD levels on the grazing of periphyton by mayflies and riffle beetles was examined.  In-stream mesocosms quantified changes in mayfly and riffle beetle grazing in streams with low, medium, and high AMD impact (n=2 per treatment).  Periphyton for the experiment were grown on tiles in a reference stream for 10 weeks before being placed inside a mesocosm with three mayflies or fifteen riffle beetles.  Twelve mesocosms, with riffle beetles, mayflies, or controls were deployed in each stream (n=4, per treatment).  Mayflies reduced periphyton biomass across all impact levels, removing the most in low impact streams and the least in medium impact streams.  Riffle beetles did not substantially decrease periphyton biomass.  Invertebrate mortality was not significant.  These data suggest that regardless of stream impact level, mayflies were more successful than riffle beetles at removing periphyton within the experimental time frame.  The interactions between periphyton and grazers under different levels of AMD impact were complex and may have been influenced by many different factors including altered flow regimes, stimulation of algal growth by increased grazing, and differences in grazer tolerance to AMD.