A complete amino acid sequence for the rubredoxin from the photosynthetic bacterium Chlorobium thiosulphatophilum is proposed. The sequence, a single polypeptide chain of 53 amino acids, was deduced from the sequences of peptides obtained by chymotryptic, tryptic, thermolytic or mild acid digestion. The rubredoxin shows a high degree of sequence homology with rubredoxins from non-photosynthetic bacteria, and the evolutionary implications of this are considered.