About 20% of the total extractable ferredoxin from a blue-green alga, Aphanothece sacrum, was separated as a minor component on a DEAE-cellulose column during the preparation of the major component of the cells. It was always found in cells collected in three different seasons. The chemical properties of the minor component was different from those of the major one. The enzymatic activity and spectral properties were those of typical chloroplast-type ferredoxins. The significance of isozymes in ferredoxin is discussed.