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Eighteen populations of Notropis lutrensis, representing all of the major river systems occupied by this species across an 1100km north-south span of its range, showed no significant differences in mean critical thermal maximum (CTM), although general north-south differences in climatic temperatures exist. There were no clinal trends in CTMs, and CTMs were unrelated to stream size or any other discernible feature of the local habitats. Previous research has shown that N. lutrensis can adapt rapidly to an artificially altered thermal regime. The present results suggest that even though a species may have potentially ‘labile’ thermal physiology, it may nonetheless be ‘static’ or evolutionarily conservative in such characters when natural populations are examined empirically.