Groundwater scientists use fluorescein dyes to trace ground water resources that supply springs which may contain threatened or endangered mollusk species. To estimate risks of a commonly used groundwater tracer to the threatened Bliss Rapids snail Taylorcocha serpenticola we tested the toxicity of sodium fluorescein solutions to a surrogate species, the ashy pebblesnail Fluminicola fuscus. Trials were conducted in static 24-h exposures to several levels of treatment as well as controls with no sodium fluorescein at temperatures similar to those in the habitat of concern. We estimated 377 mg L^-1 as the median lethal concentration for the ashy pebblesnails. Using these data, we concluded that risks to mollusks from the proposed groundwater testing were likely low.