A RELATIONSHIP exists between the thermostability of fish skeletal muscle myosin, actomyosin and myofibril preparations and the environmental temperature at which the fish lives1–3. Compared with those isolated from mammals and warm sea fishes, the myosins isolated from cold water species readily aggregate on storage, are more sensitive to denaturation by heat and urea, and quickly lose all ATPase activity following preparation4,5. We have compared the properties of myofibrils and myosin prepared from the white muscle of an Antarctic fish, Notothenia rossii, South Georgia, British Antarctica, with homologous preparations from a tropical species, Amphiprion sebea (Indian Ocean; 23–27 °C). We suggest that the thermal lability of cold-adapted fish myosins arises from differences in the higher order in the structure of the molecule; this is probably an evolutionary response to attain high catalytic efficiency at low temperatures.