The lichens Pseudoparmelia caperata (L.) Hale, Usnea of subfusca Stirt., and Umbilicaria mammulata (Ach.) Tuck. were exposed to simulated acidic rain with pH levels of 2.3, 3.0, 3.3, 4.3, or 5.6 and other ions in concentrations normally found in rain in the eastern United States. The pH levels of the most acidic treatments (3.3, 3.0, 2.3) were similar to those found in fog, cloud water, and occasional rainfall events. The pH 4.3 and 5.6 treatments compared to average ambient conditions in the eastern and western United States, respectively, and caused no significant effects on photosynthesis. However, after the first week of treatment, significant effects of rain pH at the most acidic treatments on gross photosynthesis were detected in P. caperata and U. mammulata, but not in U. of subfusca. Visible effects of injury were also observed and included bleaching, necrotic spots, and necrotic margins, which resembled damage seen in field populations of U. mammulata, the most sensitive species. (Copyright (c) 1986 by D. Reidel Publishing Company.)