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Amphibians exhibit extreme plasticity in the timing of metamorphosis, and several species have been shown to respond to water availability, accelerating metamorphosis when their ponds dry. In this study we analyzed the plasticity of the developmental response to water volume reduction in the western spadefoot toad, Scaphiopus hammondii. Also, we attempted to identify the environmental cue(s) that may signal a desiccating larval habitat. We spawned adults in the laboratory and raised tadpoles in aquaria in a controlled environmental chamber. Water levels of aquaria were gradually reduced by removing water at the rate of 0.5-1 L/d; water in control aquaria was similarly disturbed but not removed. Tadpoles subjected to water volume reduction showed significant acceleration of metamorphosis. The developmental acceleration depended on the rate of reduction of the water level; i.e., tadpoles exhibited a continuum of response. This developmental response did not result from thermal differences between treatments. Furthermore, the response was reversible in that refilling of the aquaria to the starting water level at various times following the onset of volume reduction resulted in restoration of body mass and a tendency to decelerate metamorphosis. Several lines of evidence suggest that the developmental response is due neither to the concentration of compounds in the water nor to chemical or physical interactions among conspecifics. Rather, the response appears to be related to the reduced swimming volume and perhaps the proximity to the water surface. When the water level is reduced, tadpoles reduce foraging, and food restriction of prometamorphic tadpoles maintained in a constant high water environment accelerated metamorphosis. Spadefoot toad tadpoles are a valuable model system for explaining both the proximate mechanisms (environmental cues and physiological responses) and the ultimate causes for adaptive phenotypic plasticity in amphibian metamorphosis.