The relative size and the ultrastructure of the heart ventricle of a haemoglobinless and myoglobinless teleost,Channichthys rhinoceratus, has been investigated. It is two to three times larger than that of a red‐blooded species which also lives in the Kerguelen archipelago,Notothenia magellanica.At first sight, electron micrographs of the myocardial cell do not reveal peculiar features related to the absence of respiratory pigments. A mitochondrial compartment amounting to 18% volume fractions indicates a fairly high aerobic metabolis