Ambler, R. P. (Dept. of Molecular Biology, University of Edinburgh, Edinburgh EH9 3JR, Scotland) 1974. Bacterial Cytochromes c and Molecular Evolution. Syst. Zool. 22: 554–565.—Cytochromes c are very widely distributed in bacteria, and many organisms produce several different types of protein. The bacterial cytochromes c vary very greatly in size, in number of hemes, in spectrum, in oxidation reduction potential, and in function, and these differences are reflected in great differences in amino acid sequence. In this paper, the present knowledge of the structure of bacterial cytochromes c is reviewed, and some new preliminary results presented. Sequence variation within classes of bacterial cytochromes c and relationships between cytochrome c classes are considered. The use of amino acid sequence results for the investigation of pre-Cambrian evolution is discussed, and pessimistic conclusions reached.