In organisms with complex life cycles, such as amphibians, selection is thought to have minimized the duration of metamorphosis, because this is the stage at which predation risk is presumed to be highest. Consequently, metamorphic duration is often assumed to show little if any environmentally induced plasticity, because the elevation in the extrinsic mortality risk associated with prolonging metamorphosis is presumed to have selected for a duration as short as is compatible with normal development. We examined the extent to which metamorphic duration in the anuran amphibian Xenopus laevis was sensitive to environmental temperature. Metamorphic duration was influenced by body size, but independent of this effect, it was strongly influenced by environmental temperature: the duration at 18 °C was more than double that at 24 and 30 °C. We also compared the vulnerability of larval, metamorphosing and post metamorphic Xenopus to predators by measuring their burst swimming speeds. Burst swim speed increased through development and while we found no evidence that it was reduced during metamorphosis, it did increase sharply on completion of metamorphosis. We therefore found no evidence of a substantial increase in vulnerability to predators during metamorphosis compared with larval stages, and hence the slowing of metamorphosis in response to temperature may not be as costly as has been assumed.