A lichen inventory of White Rocks Open Space (City of Boulder, CO), a unique outcropping of exposed unstable sandstones, is presented. White Rocks is an ecologically important, ∼40.5-ha (100-acre) protected area whose biological significance is in part attributable to geological and climatological history as well as degree of preservation, but also because it represents a biodiversity reservoir in a sea of agriculture and urban development. This inventory resulted in documentation of 56 species, including 1 new record to the United States, 1 new record to Colorado, 2 species new to science, and 2 additional species potentially new to science pending further study. Several species, though common at White Rocks, are locally rare within Boulder County and the Front Range Mountains of Colorado owing to a paucity of exposed sandstone. Such species instead have affinities to habitats more typical of eastern and far western Colorado, such as the state's high plains and sage-juniper scrub. A checklist of the 56 species is presented along with an assessment of taxon abundance and preliminary delimitation of 5 lichen communities. General discussion of ecology and management recommendations is provided. The present study is intended to begin the process of more fully documenting the lichen biota of Boulder City, Boulder County, and surrounding areas. Data herein also contribute new knowledge to the North American lichen biota as a whole via a comprehensive inventory of a sandstone formation, of which there exist relatively few. The inventory highlights the ecological significance of Open Space programs nationwide, particularly those in densely populated areas such as the Denver—Boulder—Longmont urban triangle. The discoveries reported here were made within a 10-min drive of the University of Colorado, demonstrating the potential for further discovery in Colorado lichenology.