Metabolic performance at various temperatures and maximal swimming speeds, critical thermal maxima and minima, as well as salinity tolerance were examined in the Mohave tui chub, Gila bicolor mohavensis. Maximal metabolic rates, ca. 0.1 cc O2/g h were attained at 24 and 30 C at swimming speeds of 87 cm/sec. At 18 C fish could attain swimming speeds of only 45 cm/sec and consumed 0.1 cc 02/g h. Factorial metabolic scope dropped from 3.3 at 18 C to 1.57 at 30 C. The mean critical thermal maxima at three acclimation temperatures (18, 24, 30 C) were 33.5, 34.9 and 36.2 C, respectively. Mean critical thermal minima were 2.8, 4.8 and 7.2 C, respectively, for fish acclimated at 18, 24 and 30 C. Mohave chubs osmoregulate well up to salinities of 237 mOsm/1. Higher salinities induce weight loss and antidiuresis. Based on these experimental results it appears that the Mohave chub is not as adaptable to desert conditions as other desert fish, such as the pupfish (Cyprinodon). Therefore, careful consideration must be given to micro-environments when new refugia are sought for this endangered species.