Cryptogamic crusts are communities composed of lichens, cyanobacteria, algae, mosses, and fungi. These integrated biological soil crusts are susceptible to disturbance, but if intact, appear to play a role in providing nutrients, especially nitrogen, to higher plants. Crust samples from the Colorado Plateau and the Great Basin were brought to the laboratory and exposed to atmospheres of different humidity and different amounts of liquid water. Metabolic heat rate (R-q) and carbon dioxide evolution rate (R-CO2) were measured in microcalorimeters at temperatures from 5 to 40degreesC. Exposure to water vapor alone had little effect, but addition of liquid water caused a marked increase in metabolic rate and a switch from anaerobic to aerobic metabolism. Crusts from both sites had maximum growth rates around 30degreesC. Colorado Plateau crusts had much higher metabolic and growth rates than Great Basin crusts. Published by Elsevier Science B.V.