The amino acid sequence of plastocyanin from the prokaryotic blue-green alga Anabaena variabilis was determined. The protein consists of a single polypeptide chain of 105 residues. The amino acid sequence of the plastocyanin was compared with that of the eukaryotic green alga Chlorella fusca and with those of higher-plant plastocyanins. The considerable similarity between the prokaryotic and eukaryotic plastocyanins is discussed. Detailed evidence for the sequence of the protein has been deposited as Supplementary Publication SUP 50051 (13 pages) at the British Library (Lending Division), Boston Spa, Wetherby, W. Yorkshire LS23 7BQ, U.K., from whom copies may be obtained on the terms given in Biochem J. (1975) 145, 5.