Eight strains of hydrogen bacteria belonging to the genera Hydrogenomonas, Pseudomonas, Nocardia, and to coryneform bacteria have been analyzed with respect to the cytochrome patterns. All strains contained cytochromes a,b,c and o. Quantitative and qualitative differences reflect the taxonomic heterogeneity of the group. Only in strain 11/x the growth conditions (auto- or heterotrophic) resulted in a qualitative difference of cytochrome content.