A monomeric flavoprotein (18.8 kDa) was isolated from the soluble cell fraction of Wolinella succinogenes and was identified as a flavodoxin based on its N-terminal sequence, FMN content, and redox properties. The midpoint potentials of the flavodoxin (Fld) at pH 7. 5 were measured as -95 mV (Fldox/Flds) and -450 mV (Flds/Fldred) relative to the standard hydrogen electrode. The cellular flavodoxin content [0.3 micromol (g protein)-1] was the same in bacteria grown with fumarate or with polysulfide as the terminal acceptor of electron transport. The flavodoxin did not accept electrons from hydrogenase or formate dehydrogenase, the donor enzymes of electron transport to fumarate or polysulfide. Pyruvate:flavodoxin oxidoreductase activity [180 U (g cellular protein)-1] was detected in the soluble cell fraction of W. succinogenes grown with fumarate or polysulfide. The enzyme was equally active with Fldox or Flds at high concentrations. The Km for Flds (80 microM) was larger than that for Fldox and for the ferredoxin isolated from W. succinogenes (15 microM). We conclude that flavodoxin serves anabolic rather than catabolic functions in W. succinogenes.