The critical thermal maxima of adult Carcinus maenas and Cancer pagurus have been determined. The CTMax for the eurythermal C. maenas were significantly higher than for the stenothermal C. pagurus at each acclimation condition, as would be predicted. Acclimation had a significant effect in both species, with 22°C acclimated crabs having significantly higher CTMax than 8°C acclimated crabs. The acclimation ability (AAR response) was greater in C. pagurus than in C. maenas, suggesting that the ability for acclimation is not directly related to eurythermicity. There was a marked seasonal effect on CTMax in both species. The CTMax of winter caught crabs was significantly lower than in summer or autumn caught animals, a difference that persisted irrespective of laboratory acclimation status.