1. The kinetics of lactate dehydrogenase (both forward and back reaction) in cardiac and skeletal muscle of an Antarctic teleost have been compared with a temperate teleost of comparable morphology and ecology.2. In both species the forward reaction (pyruvate to lactate) is maximally activated at 2.5–4 mM pyruvate and inhibited above this level.3. The Michaelis constant (Km) for pyruvate is not significantly different between muscle types or between species when measured at their normal environmental temperature.4. Km for pyruvate varies with temperature in a positive direction.5. The back reaction (lactate to pyruvate) is maximally activated by 12–16 mM lactate but only in skeletal muscle of the antarctic species is there inhibition above this level.6.6. The Km for lactate is significantly (P<0.05) lower in the Antarctic fish cardiac muscle.7.7. While the two species are morphologically and ecologically similar, differences at the biochemical level are discussed with respect to environmental temperature range and conservation of enzymic characteristics.