For various reasons lichens seem to be much more sensitive to air pollution than flowering plants. Various methods to map the long-range effect of phytotoxicants on epiphytic lichens and mosses have been proposed. This paper outlines a few of these and proposes a new method. In Sudbury, Ontario, vegetation has been greatly affected by sulfur dioxide emanating from three huge smelters. The author shows that his map based on the response of lichens matches quite well with another map from the same area based on continuous SO/sub 2/ monitoring. The advantage of the biological map is that it took two weeks to accumulate the data required while the other one took ten years.