Unproductive lakes are generally supersaturated with carbon dioxide (CO₂) and emit CO₂ to the atmosphere continuously during ice-free periods. However, temporal variation of the partial pressure of CO₂ (pCO₂) and thus of CO₂ evasion to atmosphere is poorly documented. We therefore carried out temporally high-resolution (every 6 h) measurements of the pCO₂ using an automated logger system in the surface water of a subarctic, unproductive, lake in the birch forest belt. The study period was June–September 2004. We found that the pCO₂ showed large seasonal variation, but low daily variation. The seasonal variation was likely mainly caused by variations in input and mineralization of allochthonous organic matter. Stratification depth probably also influenced pCO₂ of the surface water by controlling the volume in which mineralization of dissolved organic carbon (DOC) occurred. In lakes, with large variations in pCO₂, as in our study lake a high (weekly) sampling intensity is recommended for obtaining accurate estimates of the evasion of CO₂.