Anaerobically grown bacteria isolated from the hindgut contents of the termites Coptotermes lacteus (Froggatt), Mastotermes darwiniensis Froggatt and Nasutitermes exitiosus (Hill) were nitrogenase-positive as assayed by acetylene reduction. Nitrogen fixation, confirmed with 15N2, was highest in the isolate from M. darwiniensis. All isolates were identified as Citrobacter freundii (Braak) Werkman et Gillen.