Micrometeorological measurements of carbon dioxide and energy fluxes were made on a mesotrophic flark fen in northern Finland (69¿08'N;27¿17'E). The measurements were conducted from August 15 to September 13, 1995, the period extending from the intense phase of the growing season through its decline. The objective was to assess the net CO2 balance on a daily basis during the crucial time of the fen turning from a net sink to a net source of CO2. A clear decline in the CO2 fluxes was observed due to the senescence of the vascular plants at the end of August. The hourly median of daytime fluxes decreased from -0.12 to -0.04 mg m-2 s-1 from August to September. The nighttime fluxes decreased simultaneously from 0.07 to 0.04 mg m-2 s-1. A simple photosynthetically active radiation (PAR) driven net ecosystem exchange model was used to patch up the time series for calculating the diurnal balances. The value of the model parameter of maximal gross photosynthetic uptake changed from -0.320 to -0.134 mg m-2 s-1 from August to September. The value of α (initial slope of net ecosystem exchange versus PAR) changed, correspondingly, from -0.888 to -0.480 μg μmol-1. The highest negative daily balances at the beginning of the period were about -2 g m-2 d-1. During the first days of September they rose to 3 g m-2 d-1, but decreased to 1 g m-2 d-1 at the end of the measurements. ¿ 1998 American Geophysical Union