Pacifastacus leniusculus from two populations in Washington State, the central Columbia River and a small tributary, were acclimated at 5 C intervals and exposed to elevated temperatures in 48 hour thermal bioassays. The upper lethal temperature for both crayfish populations increased relatively slightly, from about 28.5 to 31.5 C, over the entire acclimation range. A rise of 1 C in test temperature often represented the difference between zero and total mortality when lethal limits were approached. The ultimate upper lethal temperature was near 32-33 C Statistically significant differences in thermal resistance patterns (slope and spacing of regression lines) occurred between the two crayfish populations at all acclimation levels, but resistance in terms of eventual mortality was similar for practical purposes. Moulting individuals were particularly susceptible to high temperature stress. Mature, pre-breeding female crayfish from the Columbia River during fall appeared less resistant, and egg-bearing females during winter more resistant, than other individuals. Larger crayfish from the Columbia River were slightly less resistant to elevated temperatures than smaller ones, and females were more resistant than males. The upper temperature triangle for P. leniusculus encompasses an area of 424 C2This freshwater decapod is more tolerant of elevated temperatures than native salmonids, but less tolerant than some introduced “warmwater” fish.