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The in situ search for life on Mars requires an understanding of the possible habitats available and the types of microbes that inhabit such environments on Earth. Rock varnish is ubiquitous in terrestrial deserts and has been suggested to exist on Mars. Data reported here show that there are very high numbers of bacteria (10(7)-10(8) g(-1) dry wt) associated with rock varnish collected in the hot desert of the Whipple Mountains, south of Death Valley, CA, USA. Some of the bacteria identified in the rock varnish from the Whipple Mountains are resistant to UV-C exposure. This suggests that habitats like rock varnish, if they occur in the martian polar regions where liquid water may be available, may provide niches for radiation-resistant life forms such as the bacteria observed in the Whipple Mountains varnish ecosystem. The UV-resistant microbes isolated represent a diverse group of genera, but all are from the order Actinomycetales (the genera Arthrobacter, Curtobacterium, Geodermatophilus, and Cellulomonas). They are metabolically versatile heterotrophs capable of growing on a variety of simple sugars. amino acids, organic acids and aromatic acids as sole carbon and energy sources. (c) 2005 Elsevier Inc. All rights reserved.