Red wood ants (Formica rufa group) are important elements in boreal forest ecosystems, where they occur in high abundance and build large and long-lasting, above-ground mounds of organic material. However, little is known on their role in the carbon (C) cycling in boreal forests. We measured temperature and carbon dioxide (CO₂) efflux from three different-sized wood ant mounds and the surrounding forest floor from May 2004 to April 2005 in Norway spruce [Picea abies (L.) Karst.] dominated forests in eastern Finland. Additionally, mound and forest floor temperatures were measured continuously and CO₂ effluxes at 2–4-week-intervals. During the ants’ active season (May–September), measurements were conducted in the morning, afternoon, evening and at night, while fluxes were measured once a day during the ants’ inactive season. CO₂ emissions from the mounds were up to nearly eight times higher than those from the surrounding forest floor during the active season of the ants, but no statistically significant differences were observed during the period from October to February. Both mound and forest floor CO₂ fluxes were highly correlated to mound or forest floor temperature. Based on our measurements, we are able to estimate the annual CO₂ efflux from ant mounds and the surrounding forest floor, based on nonlinear regression analyses using CO₂ flux as dependant and mound or forest floor temperatures as independent variables. Although red wood ant mounds were found to be "hot spots" for CO₂ efflux, that increase the spatial heterogeneity of C emissions within a forest ecosystem, their annual emissions were only 0.30% of that from the forest floor. Thus, our results indicate that red wood ant mounds do not directly contribute significantly to the overall C budget of the boreal forest ecosystem studied.