This paper reports on the lichen flora of the San Juan-Rio Grande National Forest located in southwestern Colorado. This survey is based on extensive collections made between 1993 and 1997, as part of an air quality bio-monitoring program and baseline. Southwest Colorado is a biologically diverse and habitat rich part of the state. Community types include arid, low-elevation shrublands, riparian communities, mid-elevation woodlands, upper elevation coniferous forests, and extensive alpine tundra. Lichen communities along the Colorado Front Range and adjacent portions of the Rocky Mountains have been studied for more than a half century; however, lichen communities in the western half of the state and particularly the southwest corner of the state are largely unknown. Based on our research we have identified a total of 316 species in 108 genera from 32 sites in southwest Colorado. Thirty-three species are reported as new records for the state; with a single new record for North America. The lichen flora for this region is diverse and well developed. Species representing all growth forms and substrates are reported. The flora is dominated by foliose species (44%), followed by crustose taxa (35%), squamulose species (14%), with fruticose species at 7%. Species were collected from all habitats and substrates including soil/moss, rock, bark/lignum, and other lichens (with 45% on rock, 25% on bark/lignum, 29% on soil/moss, and 1% on other lichens). The diversity of lichen species reported for this area reflects the complexity and variation in habitat and environmental parameters characteristic of southwest Colorado. Based on preliminary comparisons with adjacent lichen floras, in western New Mexico and southeastern Utah differences seem to be related to: 1) variation in collection intensity; 2) differences in habitat types; and 3) variation in the frequency of monsoonal summer rains.