The respiratory loss of CO2 from soil microbes beneath winter snow in forests from cold climates can significantly influence the annual carbon budget. We explored the magnitude of winter soil respiration using continuous measurements of beneath-snow CO2 concentration within the footprint of a flux tower in a subalpine forest in the Rocky Mountains. We used eddy covariance measurements from the tower to obtain estimates of total wintertime ecosystem respiration and compared them to the calculated beneath-snow CO2 flux. Soil respiration in the winter was estimated to contribute 35--48% of the total wintertime ecosystem respiration, and 7--10% of the total annual ecosystem respiration. The largest increase in soil respiration occurred in the late winter following an earlier-than-normal initiation of snowmelt and increase in snow density. Following this melt event, respiration rates increased approximately sixfold, despite an increase in soil temperature of only 0.3¿--0.5¿C. We interpret the late-winter surge in soil respiration to be triggered by a strong response of beneath-snow microbes to the pulse of meltwater coupled with extremely high sensitivity of the microbial biomass to increases in soil temperature.