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The recent discovery of cryptoendolithic microbial communities (prokaryotic, microalgal, or fungal assemblages living within structural cavities of porous rocks) in sandstone outcrops on northern Ellesmere Island in the Canadian high Arctic provides an important opportunity to explore a unique microbial habitat in one of Canada's most extreme environments. Preliminary reconnaissance in 2001 revealed a relative abundance of cyanobacterial communities in lithic environments; their presence in the subsurface in areas with limited epilithic growths appears to be controlled by the thermal capacity of the host substrate to maintain overall warmer temperatures during periods of insolation, as well as to moderate short-term temperature fluctuations. Comparisons to Antarctic cryptoendoliths reveal that Arctic microbial endoliths have much longer active growth periods resulting from an overall warmer climate.