1. Many phylogenetic lineages of animals have undergone major habitat transitions, stimulating dramatic phenotypic changes as adaptations to the novel environment. Although most such traits clearly reflect genetic modification, phenotypic plasticity may have been significant in the initial transition between habitat types. 2. Elapid snakes show multiple phylogenetic shifts from terrestrial to aquatic life. We raised young tigersnakes (a terrestrial taxon closely related to sea-snakes) in either a terrestrial or aquatic environment for a 5-month period. 3. The snakes raised in water were able to swim 26% faster, but crawled 36% more slowly, than did their terrestrially-raised siblings. A full stomach impaired locomotor performance, but snakes were less impaired when tested in the environment in which they had been raised. 4. Thus, adaptively plastic responses to local environments may have facilitated aquatic performance (and impaired terrestrial performance) in ancestral snakes as they shifted from terrestrial to aquatic existence. 5. Such plasticity may have influenced the rate or route of this evolutionary transition between habitats, and should be considered when comparing habitat-specific locomotor abilities of present-day aquatic and terrestrial species.