Cytochrome c was extracted and purified from nine species of the genusUstilago, representing five pathogen for monocotyledonous and four for dicotyledonous host species. The amino acid compositions of acid hydrolysates of cytochrome c from these species were compared and divergence values were calculated for all pairs of species. Aspartic acid, serine, glutamic acid, proline, glycine, alanine, valine, leucine, phenylalanine, lysine, and arginine were the most variable amino acids for both dicot and monocot pathogens. Differences in lysine, glutamic acid, alanine, and histidine distinguished most monocot and dicot pathogens. The dicotyledonous pathogens had fewer glutamic acid and additional alanine and histidine residues. Divergence values for cytochrome c were generally highest between species of dicotyledonous and monocotyledonous hosts and lowest for species within the monocotyledonous group. Pairs of species in the dicotyledonous group gave values only slightly lower than those for pairs with species from different groups.