Water levels in the regulated Snake River, southern Idaho, U.S.A. can fluctuate daily and seasonally due to hydroelectric demands. The federally listed threatened Bliss Rapids snail, Taylorconcha serpenticola Hershler et al., 1994 (Family: Hydrobiidae), survives in and near these fluctuation zones. Remaining T. serpenticola populations occur only in sections of the Snake River that are impacted by these hydroelectric facilities and associated springs. Because effects of rapid draw-down in fluctuation zones on T. serpenticola are unknown, we conducted a laboratory experiment to evaluate potential impacts of desiccation. Our experiment compared desiccation resistance at several air temperatures, on dry and wetted substrates, and for ‘small’ vs. ‘large’ snails. Probit regression-maximum likelihood models estimated lethal time (LT₅₀) values. Survival was significantly greater on wetted substrate than on dry substrate and was lowest at temperatures <0°C and at 37°C on dry substrate. Survival was greatest at 17°C on wetted substrate. There was no significant difference in survival at temperatures above 0°C on dry substrate other than at 37°C. LT₅₀ survival ranged from 0.5 hours at −7°C to 157.0 hours at 17°C on wetted substrate. There were no significant differences in survival relative to snail size in any treatment. Our results suggest that desiccation could impact T. serpenticola populations if snails become stranded on dry substrates during rapid water-level fluctuations of the Snake River, particularly during subzero winter or extreme high summer temperatures. The most important factor determining survival would be the ability to find refuge on the undersides of cobbles, where snails typically occur, or in habitats that remained moist for the duration of the draw-down of the river.