Abstract
SUMMARY: Cultures of N2-fixing bacteria have been isolated from the intestinal contents of humans, pigs and guinea-pigs. Acetylene-reduction tests indicate that N2-fixation may be occurring in the intestines. Best N2-fixation was by cultures provisionally identified as Klebsiella aerogenes, but other genera were also involved. All cultures fixed more N2 anaerobically than aerobically but some fixation occurred when 20% O2 was present.