Although cytochrome c genes (cyt c) and proteins (CYT C) have been relatively well studied in mammals, very little is known about them in parasitic helminths. In the present study, we investigated this group of molecules in Haemonchus contortus (barber's pole worm) and Trichostrongylus vitrinus (black scour worm), two parasitic nematodes of small ruminants. The cyt c gene (512 bp) of H. contortus had one intron and encoded a transcript of 345 nucleotides, whilst that of T. vitrinus (792 bp) had two introns and encoded a transcript of 360 nucleotides. The transcription of cyt c in T. vitrinus was substantially greater in adult males compared with females, although no such gender-enrichment was evident in adults of H. contortus. These findings were supported at the protein level by immunoblot analyses. The inferred proteins (designated Hc-CYT C and Tv-CYT C, respectively) shared nucleotide and amino acid identities of 78% and 85%, respectively. The alignment of these and other CYT C sequences from nematodes, flatworms, insects and mammals identified conserved motifs associated with CYT C oxidase- and reductase- as well as haem-binding. One residue (histidine-26) was conserved for mammals, whereas this residue was absent from all nematodes; the functional significance of this difference is not yet known. Both phylogenetic analysis and protein modelling revealed that CYT C proteins of nematodes are structurally distinct from those of mammals and other organisms, suggesting their potential as targets for parasite intervention.