Unmanaged forests at a late stage of successional development are considered to be insignificant as carbon sinks, since in theory, assimilation is thought to be balanced by respiration. However, little experimental evidence for this hypothesis exists so far for forests at the ecosystem level. Therefore, we performed continuous eddy covariance measurements of carbon dioxide over an unmanaged beech forest in the Hainich National Park in Central Germany as part of the EU project CARBOEUROFLUX. This forest shows typical characteristics of an ‘advanced’ forest with large dead wood pools, a diverse stand structure and a wide tree age class distribution, up to 250 years. This forest was a large carbon sink over 2 years, with 494gCm−2 per year in 2000 and 490gCm−2 per year in 2001. Daytime summer fluxes were strongly controlled by photosynthetic photon flux density (R 2=0.7–0.9), with minor effects of the ratio of diffuse to total downward radiation or the vapor pressure deficit. Nighttime CO2 fluxes were mainly controlled by soil temperature (R 2=0.8) and soil moisture. In addition, high nighttime CO2 fluxes (4–6μmolm−2 s−1) were found directly before and during bud break in spring as well as just after leaf fall of both years (2000 and 2001), reflecting stand physiology corresponding to phenological changes, independent of soil temperature. Additional wind profile measurements at five heights within the canopy revealed a decoupling of above and below canopy air flow under conditions of low friction velocity (u *<0.4ms−1), probably indicating down slope drainage. In conclusion, unmanaged forests at a comparatively late stage of successional development can still act as significant carbon sinks with large implications for forest management practice and negotiations about biological sinks within the Kyoto Protocol.