1. The relationship between heart ventricle weight and body weight has been determined for three species of Antarctic fish with respiratory pigments (Notothenia gibberifrons, Notothenia neglecta and Notothenia rossii) and the haemoglobin-less icefish Chaenocephulus aceratus.2.2. Relative heart weights of Notothenidae are similar to those of other teleosts whilst those of Channichthyidae are similar to those reported for tuna and small mammals.3.3. The volume densities of mitochondria and myofibrils for ventricular myocytes were 0.47 and 0.25 respectively. The values for mitochondrial volume density are higher and those for myofibrillar volume density lower than for most vertebrate hearts.4.4. Some unusual characteristics of these mitochondria are reported and discussed in relation to the unique constraints characterizing this type of heart.