Carbon dioxide (CO2) concentration under snow was examined through two winter seasons at a 3100 m elevation subalpine site in the Snowy Range of Wyoming. CO2 was monitored every half hour at the soil/snow interface, and at about 25 cm soil depth the second year, in a meadow and in an adjacent forest. CO2 under snow in the meadow was significantly higher than that in the forest. CO2 at 25 cm depth in the soil was significantly higher than soil surface CO2. The CO2 under snow increased rapidly as snow melted and snowmelt began in the spring. CO2 concentration under snow depended primarily on amount occurring during the previous 24 or 48 h. However, CO2 concentration was related to snow depth and soil temperature, and indirectly to several seasonal environmental factors, especially solar radiation. Solar radiation, snow depth, and CO2 under snow all increase concurrently as the winter season progresses. CO2 flux was consistently higher in the meadow than in the forest and increased in late winter for both sites. Snow covered subalpine meadows and forests contribute considerable amounts of CO2 to the atmosphere in the winter.