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Sandstone outcrops around Eureka, Ellesmere Island, Nunavut (80 degrees N) in the Canadian high Arctic are host to abundant cryptoendolithic microbial communities. Continuous measurements over 2 years (2002-2004) of climate and environmental parameters showed that cryptoendolithic habitats experience warmer temperatures and wetter conditions than the exposed rock surface. Subsurface temperature fluctuations were moderated by the thermal capacity of the rock substrate and varied as a function of depth, aspect, and albedo. Rain, snow or snowmelt substantially increased subsurface moisture levels, which persist for significant time periods after initial precipitation events. These conditions produced a habitat amenable to colonization by cyanobacteria, fungi and algae. The dominance of one microbial community over another varied between sites, however these differences existed in habitats with similar temperature conditions. Greater diversity of microorganisms at this Arctic location compared to similar habitats in the Antarctic Dry Valleys is explained by warmer temperatures during summer months that lead to longer periods for both active (similar to 3,700 h year(-1)) and ideal (similar to 2,500 h year(-1)) calculated metabolic activities as well as abundant moisture in the form of liquid water.