Anaerobic nitrogen fixation in the dark was determined in a natural marine community, developed in a laboratory model ecosystem, at the Oregon State University Marine Science Center in Newport, Oregon. Fixation occurred at a rate of 10 nmol C2H4/cm2per 24 hours. Two different marine N2-fixing bacteria were isolated from seawater and from sediment at the Oregon coast. These bacterial isolates were identified asKlebsiella pneumoniaeandEnterobacter aerogenes. Within a growth period of 100 hK.pneumoniaefixed 3.6 mmol N2per liter andE.aerogenesfixed 1.2 mmol N2per liter. The influence of seawater on N2-fixation activity of these bacteria was compared with that of bacteria from a non-marine origin. The nitrogenase activity of a crude cell-free extract ofE.aerogeneswas dependent upon ATP and Na2S2O4and proceeded at a rate of 3.9 nmol C2H4per minute per milligram protein.