Acetylene reduction by bacteria in the rhizosphere of Glycerin borealis and Typllrc sp. was studied under anaerobic and aerobic conditions. Under anaerobic conditions, when roots and rhizomes were incubated with rhizosphere sediments at 25'C without glucose, acetylene reduction to ethylene was calculated to be 37 pmol g-1 day-1 and 3.6 pmol g-1 day-1 for Glyceria and Typlla respectively. The rates were generally lower under aerobic conditions. Washed roots (without sediment) also showed high activity. The reduction of acetylene to ethylene had a Q10 of 4.5 in the Glyceria system. It is estimated that N2 fixation in the rhizosphere of this species may reach 60 kg ha-1 year-1.