In comparison with collections from the early 1900's when oxidant air pollution was essentially absent, 50% fewer lichen species were found on conifers during 3 yr (1976-1979) of col- lecting and sampling in the mountains of Southern California. Among the five mountain ranges studied, the San Bernardino Mountains, the region with the highest oxidant levels, had lower lichen frequency and cover values. Within the San Bernardino study sites, lichen cover was inversely related to estimated oxidant doses. Furthermore, at sites with high oxidant levels, marked morphological de- terioration of the common species Hypogymnia enteromorpha was documented. Transplants of this species from the relatively unpolluted Cuyamaca Rancho State Park into the San Bernardino Moun- tains exhibited similar deterioration after a year's exposure. Key words: community structure; conifers; differential sensitivity; lichens; oxidant air pollution; San Bernardino Mountains; smog; Southern California.