The Fe protein of Anabaena cylindrica was first separated and purified by chromatography through DEAE-cellulose columns then by gel electrophoresis. The specific activity was up to 142.46 nmol C2H4/mg protein .cntdot. min. It was homogeneous as shown by (1) a single band in the gel electrophorogram; (2) absence of Mo and tryptophan; (3) content of about 3.4 atoms of Fe per mole protein. The molecular weight of the Fe protein of A. cylindrica was about 61,000 daltons as estimated by SDS-gel electrophoresis and calculated from the amino acid composition. The residues of aspartate and glutamate were about 2.6 times that of arginine and lysine in the Fe protein. Crossing Fe protein of A. cylindrica with Mo-Fe protein of Azotobacter vinelandii gave positive result. The reciprocal crossing also showed activity.