Antarctic fish from the suborder Notothenioidei inhabit what is perhaps the most thermally stable ocean environment on Earth. Evolutionary theory suggests that by specialising for this environment, Antarctic fish have traded-off their ability to respond to variations in temperature, and like their environment, have become extremely stenothermal. However, previous research has revealed that the Antarctic notothenioid fish Pagothenia borchgrevinki is not as thermally limited as evolutionary theory might predict, and is capable of acclimation to 4 ℃ during a one month period. The purpose of the current research was to investigate the physiological mechanisms that underpin this remarkable acclimatory ability. P. borchgrevinki were acclimated for one month to 4 ℃ and changes in oxygen consumption, prolonged swimming ability, cardiovascular function, enzyme activity and haematology were measured. Significant changes in resting oxygen consumption rate and prolonged swimming ability occurred during the acclimation period, and these changes were mediated by adjustments of enzyme activity and specific aspects of the haematology. By monitoring resting oxygen consumption and prolonged swimming ability over a much longer, six month, acclimation period it was confirmed that the adjustments evident during one month at 4 ℃ were sustainable in the long-term, and were not short-term compensatory mechanisms. Interestingly, fish infected with x-cell gill disease did not possess the same ability to acclimate as was demonstrated by healthy P. borchgrevinki. P. borchgrevinki are unusual among the notothenioids, possessing an active, pelagic lifestyle which differs from the sedentary, benthic lifestyle of most other species within the suborder. Therefore, it was hypothesised that the acclimatory ability demonstrated by this species may also be unusual among the notothenioids. To test this hypothesis, the acclimation ability of three sedentary, benthic notothenioids (Trematomus bernacchii, T. hansoni and T. pennellii) was investigated. Results confirmed the hypothesis, with all three species demonstrating very poor survival at 4 ℃ and absolutely no capacity for acclimation. Such results present a disturbing scenario for the future of Antarctic notothenioid fish in Earth?s rapidly warming climate, and highlights the need for continued research combined with immediate action to combat the warming which currently threatens Antarctic marine biodiversity.