Streamside measurements of critical thermal maxima (T crit ), swimming performance (U crit ), and routine (R r ) and maximum (R max ) metabolic rates were performed on three populations of genetically distinct redband trout Oncorhynchus mykiss in the high-desert region of south-eastern Oregon. The T crit values (29Á4 AE 0Á1 C) for small (40-140 g) redband trout from the three streams, and large (400-1400 g) redband trout at Bridge Creek were not different, and were comparable to published values for other salmonids. At high water temperatures (24-28 C), large fish incurred higher metabolic costs and were more thermally sensitive than small fish. U crit (3Á6 AE 0Á1 L F s À1 ), R r (200 AE 13 mg O 2 kg À0Á830 h À1 ) and metabolic power (533 AE 22 mg O 2 kg À0Á882 h À1 ) were not significantly different between populations of small redband trout at 24 C. R max and metabolic power, however, were higher than previous measurements for rainbow trout at these temperatures. Fish from Bridge Creek had a 30% lower minimum total cost of transport (C min ), exhibited a lower refusal rate, and had smaller hearts than fish at 12-mile or Rock Creeks. In contrast, no differences in U crit or metabolism were observed between the two size classes of redband trout, although C min was significantly lower for large fish at all swimming speeds. Biochemical analyses revealed that fish from 12-mile Creek, which had the highest refusal rate (36%), were moderately hyperkalemic and had substantially lower circulating levels of free fatty acids, triglycerides and albumin. Aerobic and anaerobic enzyme activities in axial white muscle, however, were not different between populations, and morphological features were similar. Results of this study: 1) suggest that the physiological mechanisms that determine T crit in salmonids are highly conserved; 2) show that adult (large) redband trout are more susceptible to the negative affects of elevated temperatures than small redband trout; 3) demonstrate that swimming efficiency can vary considerably between redband trout populations; 4) suggest that metabolic energy stores correlate positively with swimming behaviour of redband trout at high water temperatures; 5) question the use of T crit for assessing physiological function and defining thermal habitat requirements of stream-dwelling salmonids like the redband trout.