Increased temperature as a potentially threatening process, and the need to investigate the thermal tolerance of the 'highland-rainforest' Euastacus were first identified 20 years ago; however, the thermal repertoire of Euastacus has still not been explored. Euastacus is the largest of Australia's 10 freshwater crayfish genera with 52 species, and includes many of the largest, slowest-growing and longest-lived species (some >35 years) in the World. Several species have distributions consistent with being 'climate refugees', namely, being closely associated with cool, damp conditions and restricted to isolated mountain-top refuges. The present study investigated the critical thermal limit of a well known abundant species, Euastacus sulcatus, from central eastern Australia. Thermal limit was assessed using chronic, ongoing exposure to steadily increasing temperature, with the breakdown of physiological function tested by righting response. Distress was clearly evident in the crayfish at ~23à(e.g. sluggish, lack of aggression), and the test criterion was met at ~27ì with animals effectively incapacitated and unable to right themselves. Field water temperatures rarely exceed 21û however, any increases in environmental temperature may expose this species to temperatures where physiological stress may become problematic.