An understanding of the proximate (morphological, physiological and behavioural) mechanisms that affect locomotor speed can clarify reasons for intraspecific and interspecific variation in this ecologically relevant performance measure. To explore such mechanisms, we selected a study system involving animals with highly simplified external morphology (snakes) moving through a homogeneous medium (water). In laboratory trials, swimming speeds in two species of amphibious sea kraits (61 Laticauda laticaudata and 35 L. saintgironsi) varied from 0.14 to 1.39 body lengths per second. Miniature accelerometers attached externally to these snakes recorded the frequency and lateral acceleration of bodily undulations that the snakes used to propel themselves through the water. Frequency of undulation was constant along the length of the body, but amplitude of lateral acceleration increased in posterior sections. The frequency and amplitude of lateral acceleration of undulations were highly correlated with swimming speeds, suggesting (a) that intraspecific and interspecific variation in swimming speeds reflects underlying variation in undulatory mechanics; and (b) that data from miniature accelerometers implanted in the body cavity could provide accurate estimates of swimming speeds of free-ranging snakes.