A chioroplast-type ferredoxin was isolated and purified from a blue-green alga, Aphanothece sacrum (Suringar) OKADA. The purified ferredoxin had absorption maxima at 462, 422, 330, and 275 nm in its oxidized form. Two atoms each of non-heme iron and labile sulfur were found per mole of the protein. It was active in the NADP+ photoreduction system of spinach chloroplasts and is similar to spinach ferredoxin. It also showed behaviour similar to that of spinach ferredoxin in complex formation with ferredoxin-NADP reductase [EC 1.6.7.1] isolated from spinach leaves. The amino acid composition of Aphanothece ferredoxin was Lys4, His1, Arg1, Asx14, Thr7, Ser6, Glx12, Pro6, Gly7, Ala9, Cys4, Val6, Met0, Ile5, Leu8, Tyr6, Phe1, Trp0, Only four cysteine residues were found as compared with either five or six in many other chloroplast-type ferredoxins. Nearly half of the total amino acid sequence was determined by means of a sequence analyzer. A valine residue at the 18th position was found in Aphanothece ferredoxin instead of the cysteine residue common in other ferredoxins. It was therefore proposed that the four cysteine residues necessary to conform the iron-sulfur cluster in chloroplast-type ferredoxin were probably those at positions corresponding to 39, 44, 47, and 77 in the sequence of spinach ferredoxin. The carboxyl-terminal sequence was Ala-Leu-Tyr, unique among ferredoxins so far sequenced.