<jats:p>Mitochondrial changes such as tight coupling of the mitochondria have facilitated sustained oxygen and respiratory activity in haemoglobin-less icefish of the Channichthyidae family. We aimed to characterise features in the sequence and structure of the proteins directly involved in proton transport, which have potential physiological implications. ATP synthase subunit a (<jats:italic>ATP6</jats:italic>) and subunit 8 (<jats:italic>ATP8</jats:italic>) are proteins that function as part of the F<jats:sub>0</jats:sub> component (proton pump) of the F<jats:sub>0</jats:sub>F<jats:sub>1</jats:sub>complex. Both proteins are encoded by the mitochondrial genome and involved in oxidative phosphorylation. To explore mitochondrial sequence variation for ATP6 and ATP8 we analysed sequences from <jats:italic>C</jats:italic>. <jats:italic>gunnari</jats:italic> and <jats:italic>C</jats:italic>. <jats:italic>rastrospinosus</jats:italic> and compared them with their closely related red-blooded species and eight other vertebrate species. Our comparison of the amino acid sequence of these proteins reveals important differences that could underlie aspects of the unique physiology of the icefish. In this study we find that changes in the sequence of subunit a of the icefish <jats:italic>C</jats:italic>. <jats:italic>gunnari</jats:italic> at position 35 where there is a hydrophobic alanine which is not seen in the other notothenioids we analysed. An amino acid change of this type is significant since it may have a structural impact. The biology of the haemoglobin-less icefish is necessarily unique and any insights about these animals will help to generate a better overall understanding of important physiological pathways.</jats:p>