Estimates of annual carbon loss from arctic tundra ecosystems are based nearly entirely on measurements taken during the growing season in part because of methodological limitations but also reflecting the assumption that respiration during winter is near zero. Measurements of CO2 flux during winter, however, indicate significant amounts of carbon loss from tundra ecosystems throughout the 240-day nongrowing season. In our study during the 1996 and 1997 nongrowing seasons, winter carbon losses ranged from 2.0gCO2m-2season-1 in moist dwarf shrub communities to 97gCO2m-2season-1 in natural snowdrift communities, with an average wintertime CO2 efflux of 45gCO2m-2 for all Low Arctic tundra communities (0.14PgCO2yr-1 worldwide). These measurements indicate that current estimates of annual carbon loss from tundra ecosystems are low. Inclusion of wintertime losses of CO2 into annual carbon budgets increases the annual carbon efflux of arctic tundra ecosystems by 17% and changes some ecosystems from net annual sinks to net sources of CO2 to the atmosphere. ¿ 1999 American Geophysical Union