The actual and potential denitrification in an alder swamp was measured in situ. A bottle method with inserted serum bottles without bottoms was used to trap N₂O escaping from the soil. Acetylene was added to the head-space as inhibitor to stop denitrification at N₂O. Measurements were performed monthly and N₂O was usually collected over a 24 hour period. Denitrification rates increased with addition of nitrate and acetate and denitrification continued for at east 48 hours. The annual denitrification rate was calculated from these monthly measurements. The denitrifying activity was very low in the winter-season and highest in spring and summer. The annual denitrification rate in alder soil was 4.9 kg N ha^-1 with acetylene as inhibitor and 42.1 kg N ha^-1 when also acetate and nitrate was added.