The amino acid sequence of a ferredoxin from a thermoacidophilic archaebacterium, Sulfolobus acidocaldarius, was determined by a combination of various conventional methods to be as follows: Gly-Ile-Asp-Pro-Tyr-Arg-Thr-His-Lys-Pro-Val-Val-Gly-Asp-Ser-Ser-Gly-His- Lys-Ile -Tyr-Gly-Pro-Val-Glu-Ser-Pro-Lys(Me)-Val-Leu-Gly-Val-His-Gly-Thr-Ile-Val -Gly-Va l-Asp-Phe-Asp-Leu-Cys-Ile-Ala-Asp-Gly-Ser-Cys-Ile-Thr-Ala-Cys-Pro-Val-As n-Val-P he-Gln-Trp-Tyr-Glu-Thr-Pro-Gly-His-Pro-Ala-Ser-Glu-Lys-Lys-Ala-Asp-Pro-V al-Asn- Glu-Gln-Ala-Cys-Ile-Phe-Cys-Met-Ala-Cys-Val-Asn-Val-Cys-Pro-Val-Ala-Ala- Ile-Asp -Val-Lys-Pro-Pro. It was composed of 103 amino acid residues giving a molecular weight of 10,908 excluding Fe and S atoms. This ferredoxin contained an N6-monomethyllysine residue at position 29 which was determined by a comparison of the elution profile of the acid hydrolysates of the protein and peptides on an amino acid analyzer with three methyl derivatives of lysine and also by field desorption mass spectrometry of a purified peptide. The ferredoxin has only 7 cysteine residues, which probably participate in constructing the Fe-S clusters of this ferredoxin, indicating the presence of a unique chelate structure. Comparison of this ferredoxin with other archaebacterial ferredoxins indicated that the archaebacteria might have multiple origins in an evolutionary tree.