Abstract Warm climates should favour aerobic N 2 fixation in plant litters because the consequent higher rates in microbial consumption of O 2 tend to reduce O 2concentrations in the vicinity of diazotrophs, thereby protecting nitrogenase against deleterious effects of O 2 This was tested in experiments with two litter systems “sugarcane culture” (wheat straw inoculated with a crude culture from sugarcane litter) and Spartina alterniflora litter from a temperate saltmarsh. Both systems exhibited maximal nitrogenase activity (assayed by acetylene reduction) in the region of 24–27 °C, but sugarcane culture exhibited a much broader temperature response. Consistent with our hypothesis, the nitrogenase activity under higher O 2 concentrations (21 and 50% O 2) was inhibited more than activity at 5% O 2 by incubating litter systems at sub-optimal temperature (12 C). CO 2 production in sugarcane culture declined by smaller factors in response to lowered temperature than did nitrogenase activity. Marsh litter exhibited high nitrogenase activity only in a water-saturated stale (83% water). Sugarcane culture exhibited high activity at much lower moisture contents. When the sugarcane cultures were made anaerobic and amended with glucose. only those that had been previously incubated under waterlogged conditions exhibited high nitrogenase activity. This indicates that the high nitrogenase activity observed in sugarcane culture incubated under air at the lower moisture contents representative of upland conditions was strictly aerobic in nature.