Photochemical transformations are important processes in the production and distribution of carbon monoxide (CO) in lakes and oceans. Initial studies of dissolved CO concentrations, CO fluxes, and in situ photochemical production were conducted at a boreal forest beaver pond near Thompson, Manitoba. Dissolved CO concentration profiles are characterized by stronger vertical gradients during the summer months. Midday integrated CO content of the upper water column ranges from 5--6 μmol/m2 in the summer to 3--4 μmol/m2 in the autumn. Surface concentrations of dissolved CO increase after sunrise in both summer and fall, attaining up to 120 nmol/L by late afternoon in the summer compared with 50 nmol/L for the same time in the autumn. The CO supersaturation ratio for this pond ranges from 37 to 875, which is 4--7 times higher than other freshwater systems studied. CO flux across the air-water interface, measured in static floating quartz chambers, tracks well with UV-A, UV-B, and full spectrum solar irradiance in the summer. The maximum flux measured at solar noon of 1.6 nmol/m2/s on Julian day 168 in June 1994 was 5 times higher than that measured at the same time of day and location on Julian day 269 in September 1995. The depth to which photochemical production of CO occurs is limited in these dark waters because of the strong absorbance of solar radiation by dissolved organic matter. In situ exposure experiments, conducted over 24 daylight hours, under autumn light conditions, indicated that up to 20 nmol of CO can be produced photochemically at 5 cm depth from 1 mg of dissolved organic carbon, rapidly decreasing to less than one fourth that amount below 7 cm depth. In situ photoproduction rates compare favorably with those from a theoretical model (GCSOLAR). Measured static fluxes are similar to what would be expected from a simple laminar film diffusive flux model given the measured supersaturation ratios or the in situ photoproduction rates as input values. ¿ 1997 American Geophysical Union