Carbon dioxide concentrations through snow were measured in different arctic tundra communities on the North Slope of Alaska during winter and early spring of 1996. Subnivean CO₂ concentrations were always higher than atmospheric CO₂. A steady state diffusion model was used to generate conservative estimates of CO₂ flux to the atmosphere. The magnitude of CO₂ efflux differed with tundra community type, and rates of carbon release increased from March to May. Winter CO₂ efflux was highest in riparian and snow bed communities and lowest in dry heath, upland tussock, and wet sedge communities. Snow generally accrues earlier in winter and is deeper in riparian and snow bed communities compared with other tundra communities, which are typically windswept and do not accumulate much snow during the winter. These results support the hypothesis that early and deep snow accumulation may insulate microbial populations from very cold temperatures, allowing sites with earlier snow cover to sustain higher levels of activity throughout winter compared to communities that have later developing snow cover. Extrapolating our estimates of CO₂ efflux to the entire snow-covered season indicates that total carbon flux during winter in the Arctic is 13–109 kg CO₂-C ha⁻¹, depending on the vegetation community type. Wintertime CO₂ flux is a potentially important, yet largely overlooked, part of the annual carbon cycle of tundra, and carbon release during winter should be accounted for in estimates of annual carbon balance in arctic ecosystems.