The complete primary structure of rubredoxin (Rd) isolated from Clostridium perfringens was sequenced to be: MICKFICDVCGYIYDPAVGDPDNGVEPGTEFKDIPDDWVCPLCGVDKSQFSETEE. The sequence was highly homologous to that of C. pasteurianum Rd but was different at 13 sites out of the total 54 amino acid residues (76% homology). It contained 1Fe atom, 4 cysteine residues, and no labile sulfur, had a molecular weight of 6, 056, and shared the general properties of classical anaerobic Rds. The pI was 4.4. The Rd was reduced with NADH in the presence of a specific NAD (P) H oxidoreductase preparation from the bacterium. The K_m value of nitrate reductase for Rd as an electron-donor was 12 μM, a value comparable to that of the 13 μM for ferredoxin (Fd). These results taken together provide additional support for its role as the electron carrier in the nitrate reductase system [Seki, S., Ikeda, A., and Ishimoto, M.(1988) J. Biochem. 103, 583-584].