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Cryptogamic crusts are communities composed of lichens, cyanobacteria, algae, mosses, and fungi. These integrated soil crusts are susceptible to disturbance, but if intact, appear to play a role in providing nutrients, especially nitrogen, to higher plants. It is not known how or under what conditions desert crusts can grow. Crust samples from localities on the Colorado Plateau and the Great Basin were brought to the laboratory and exposed to atmospheres of different humidity and different levels of liquid water. Both metabolic heat rate (q) and carbon dioxide evolution rate (R-CO2) were measured in microcalorimeters at temperatures from 10 to 35 degreesC. While exposure to water vapor alone had little effect, addition of liquid water caused a marked increase in metabolic rate and a switch from anaerobic to aerobic metabolism.