Benthic algal assemblages were surveyed in five Pennsylvania streams in order to examine trends in biomass and taxonomic groups along an acidification gradient. Acidification severity of each stream was estimated from total dissolved aluminum (Al_TD), and discharge regression equations were established for each stream using historic data from the EPA Episodic Response Program. Benthic algae were sampled once after an acidic episode in April and again during August base flows. Chlorophyll decreased with increasing episodic severity in April, thus acid episode severity and associated aluminum toxicity may have influenced algal biomass. Cyanophyta biovolume decreased with increasing severity, while Bacillariophyta, Chlorophyta, and Rhodophyta showed no distinct patterns. A diatom, Eunotia exigua, exhibited a parabolic response in April over the Al_TD gradient. Episodic acidification severity may influence algal biomass and composition in streams.