Carbon dioxide efflux and soil microenvironment were measured in three upland tundra communities in the foothills of the Brooks Range in arctic Alaska to determine the magnitude of CO2 efflux rates and the relative importance of the belowground factors that influence them. Gas exchange and soil microenvironment measurements were made weekly between 14 June and 31 July 1990. The study communities included lichen-heath, a sparse community vegetated by lichens and dwarf ericaceous shrubs on rocky soils, moist Cassiope dwarf-shrub heath tundra, dominated by Carex and evergreen and deciduous shrubs on relatively deep organic soils, and dry Cassiope dwarf- shrub heath of stone-stripe areas, which was of intermediate character. Rates of CO2 efflux were similar for the three communities until mid- season when they peaked at rates between 4.9 and 5.9 g m-2 d-1. Following the mid-season peak, the rates in all three communities declined, particularly in the lichen-heath. Seasonal patterns of CO2 efflux, soil temperature, and soil moisture suggest changing limitations to CO2 efflux over the course of the season. Rates of carbon dioxide efflux followed changes in soil temperature early in the season when soil moisture was highest. Mid-season efflux appeared to be limited by soil, moss, and lichen hydration until the end of July, when temperature again limited efflux. Differences between the communities were related to microenvironmental differences and probable differences in carbon quality. The presence of peat-forming mosses is suggested to play an important role in differences in efflux and microenvironment among the communities.