The species richness and abundance of corticolous lichens was studied as a function of distance from an anthropogenic edge, in three forest types. Lichens and light were sampled systematically in transects from the edge to 45 m into the interior, in natural forests of Pinus resinosa, P. banks iana and in hardwood forests in NW Minnesota, and in two hardwood forests 100 km from New York City. Three-fold greater species richness per tree in MN, compared to the East is likely due to air pollution. Light levels were highest at the edge in forests with recent (4 yr.old) clearcuts, but in the two forests with historical edges, the edge was occluded by dense vegetation, and light was heterogeneous. Edge effects, which varied with the species of lichen and with the compass orientation, were most pronounced for crusts and were often not explained by light levels, using statistical tests. Differences in lichen flora between stands with and without edges showed that edge effects extended into the forest for more than 45 m