Abstract The effects of aerobic and anaerobic conditions on nitrogenase activities in forest soils (in particular that of a beech forest) obtained from northeast Bavaria (Germany) were assessed by both C 2 H 2 -reduction and 15 N 2 -fixation methods. Nitrogenase activity occurred without detectable delay under both aerobic and anaerobic conditions; in both cases, initial rates approximated 0.2 nmol C 2 H 2 reduced g −1 d.w. soil d −1 . Prolonged anaerobic conditions yielded rates approximating 6 nmol C 2 H 2 reduced g −1 d.w. soil d −1 ; in contrast, rates remained relatively constant under aerobic conditions independent of the incubation period. When soils were amended with glucose, significantly higher C 2 H 2 -reduction rates (1–4 μmol C 2 H 2 reduced g −1 d.w. soil d −1 ) occurred only under anaerobic conditions. Assays with 15 N 2 confirmed this N 2 -fixation activity; based on initial reaction rates with moistened, but otherwise unamended soils, estimated N 2 -fixation activity approximated 0.2 kg N-fixed ha −1 y −1 . Compared to activities obtained under static conditions, shifts from aerobic to anaerobic conditions greatly enhanced N 2 -fixation activities. High anaerobic-to-aerobic activity ratios were obtained with both pH neutral and weakly acidic (pH 4.5–5.9) hardwood and coniferous forest soils. These consistently high anaerobic N 2 -fixation activities indicate that this process is likely limited to O 2 -deficient microsites in forest soils. No nitrogenase activity was detected with extremely acidic coniferous forest soils (pH 2.7–3.3) under either anaerobic or aerobic conditions.