After exposure to simulated acid rain at pH levels from 2.7 to 3.4, lesions of several types were produced on foliage of six clones of Populus spp. hybrids. The types of adaxial leaf surface lesions were observed at low magnification by scanning electron microscopy, and via leaf histology. On two clones, galls resulting from hyperplasia and hypertrophy of parenchyma cells predominated. In contrast, two clones exhibited neither hyperplasia nor hypertrophy. In these clones the upper epidermis, palisade parenchyma, and sponge parenchyma were injured in succession. In a third set of two clones, hyperplasia and hypertrophy were present in areas between injured and apparently noninjured tissues. In general, percent leaf area with lesions and percent leaves injured were similar among all six clones at all pH levels tested. At pH 2.7 up to 10% of the leaf area was injured after 5 daily exposures of 6 min each. Injury decreased to about 1.0% at pH 3.4. Lesions developed mostly near stomata and prior to maximum leaf enlargement. Very young and older leaves were less affected. The results support the hypothesis that the adaxial leaf surface is the most affected after exposure to simulated acid rain.