The Arkansas darterEtheostoma craginiis declining and now occurs in fragmented populations across its former range in the Arkansas River basin from Colorado to Arkansas. Regional surveys indicate that the Arkansas darter primarily inhabits small spring streams with dense aquatic vegetation, suggesting that specific habitat requirements may account for its widespread decline. Thermal tolerance and vegetation preference of the Arkansas darter from the western edge of its range in Colorado were measured in the laboratory and compared with those of the johnny darterE. nigrum, a more abundant habitat generalist from an adjacent basin. Critical thermal maxima of Arkansas darters acclimated to 20, 25, 27.5, and 30°C were higher than those forjohnny darters (35.0–38.4°C versus 34.0–37.4°C). In a modified upper incipient lethal temperature test, Arkansas darters acclimated to 27.5°C all survived 106 h at 30°C when the temperature was raised 1.5°C/h, but almost none survived more than 4 h of exposure to 34 or 36°C. Sixty percent survived 106 h at 32°C. In contrast, all johnny darters died within 17.5 h when exposed to 32°C and within 4.5 h when exposed to 34°C or higher. In behavioral choice trials in aquaria, Arkansas darters selected dense vegetation over open sand 72% of the time, with higher use during day (90%) than night (60%). In contrast, johnny darters showed little preference for vegetation (51% overall). Arkansas darters of the sizes we tested (28–55 mm total length) were able to withstand higher water temperatures than johnny darters (43–67 mm) and many other fishes of Great Plains streams, and they have an innate preference for aquatic vegetation.