Antarctic fish of the family Channichthyidae (Icefishes) lack the respiratory pigments haemoglobin and myoglobin. The morphometrics and ultrastructure of the ventricular myocardium of a benthic icefish,Chaenocephalus aceratus has been compared with that of a red-blooded Notothenioid fish,Notothenia neglecta, of similar habit.The mass of ventricular muscle as a percentage of bodyweight is 3 times greater in adultC. aceratus (0.32%) thanN. neglecta (0.11%). Myoglobin concentration in the ventricle ofN. neglecta, 20 nmoles/g, is comparable to that of temperate teleosts with similar activity patterns. The volume and surface densities of mitochondria are 41.5% and 0.32 μm(-1) for Icefish and 25% and 0.15 μm(-1) forN. neglecta, Cytochrome oxidase activities are similar in the two tissues whilst the volume density of myofibrils is higher forN. neglecta (47%) thanC. aceratus (29.9%).The proliferation of mitochondria in the myocardium of Icefish will reduce the diffusion path-length for oxygen between ventricular lumen and the outer mitochondrial membrane and may compensate for the absence of myoglobin.