A flavoprotein fromAzotobacter vinelandii, named in the literature Shethna flavoprotein or azotoflavin, has recently been shown to catalyze acetylene reduction by a cell-free nitrogenase preparation fromAzotobacter. In this communication this result is confirmed. Moreover, the Shethna flavoprotein is now shown to replace ferredoxin in the catalysis of NADP+-reduction by illuminated spinach chloroplasts or by molecular hydrogen and hydrogenase fromClostridium. Evidence is presented that the fully reduced form of the Shethna flavoprotein is involved in the catalysis of NADP+-reduction. This indicates that the Shethna flavoprotein functions as a substitute for an one electron carrier shuttling between fully reduced and semiquinoid form. From the data reported here, together with that in the literature, it is concluded that this flavoprotein belongs to the class of flavodoxins. Some preliminary results on ferredoxin fromAzotobacter are reported.