To test the temperature sensitivity of molecular chaperones in poikilothermic animals, we purified the molecular chaperone Hsc70 from 2 closely related notothenioid fishes—the Antarctic species Trematomus bernacchii and the temperate New Zealand species Notothenia angustata—and characterized the effect of temperature on Hsc70 adenosine triphosphatase (ATPase) activity. Hsc70 ATPase activity was measured using [α-³²P]-adenosine triphosphate (ATP)–based in vitro assays followed by separation of adenylates by thin-layer chromatography. For both species, a significant increase in Hsc70 ATPase activity was observed across a range of temperatures that was ecologically relevant for each respective species. Hsc70 from T bernacchii hydrolyzed 2-fold more ATP than did N angustata Hsc70 at 0°C, suggesting that the Antarctic molecular chaperone may be adapted to function more efficiently at extreme cold temperatures. In addition, Q₁₀ measurements indicate differential temperature sensitivity of the ATPase activity of Hsc70 from these differentially adapted fish that correlates with the temperature niche inhabited by each species. Hsc70 from T bernacchii was relatively temperature insensitive, as indicated by Q₁₀ values calculated near 1.0 across each temperature range measured. In the case of Hsc70 purified from N angustata, Q₁₀ values indicated thermal sensitivity across the temperature range of 0°C to 10°C, with a Q₁₀ of 2.714. However, Hsc70 from both T bernacchii and N angustata exhibited unusually high thermal stabilities with ATPase activity at temperatures that far exceeded temperatures encountered by these fish in nature. Overall, as evidenced by in vitro ATP hydrolysis, Hsc70 from T bernacchii and N angustata displayed biochemical characteristics that were supportive of molecular chaperone function at ecologically relevant temperatures.