We measured CO₂ concentration in a deciduous forest canopy and compared it with wind speed, net radiation and soil respiration. On a clear day, minimum CO₂ concentration was observed at the height where the canopy was the densest. Maximum concentration was observed just above the ground before sunrise on a calm day. The concentration tended to be higher near the ground than near the top of the canopy on all days and this concentration gradient was steeper at night, resulting from the high respiration rate of the soil and from the small transport of CO₂ out of the canopy due to low wind. This suggests that the respiration rate of the soil should be considered when measuring assimilation and respiration rate of plants by the aerodynamic method.