Cytochrome c fromAspergillus niger consists of two forms, a major one (80%) with 111 amino acid residues and a minor one (20%) with 108 residues, missing the three N-terminal residues of the major one. The primary sequence ofA. niger cytochrome c was determined by standard spinning-cup Edman degradation of purified peptides and of pairs of peptides, from which the desired sequence was readily deduced by subtraction of common sequencies. Except for the extension and some variability at the N-terminal sequence, theA. niger protein conforms well with other cytochrome c structures.