Description of Pachypleurosaurus edwardsi, from the middle Triassic of Italy and Switzerland, provides a basis for establishing the pattern of aquatic locomotion within the Nothosauria and for determining their relations with more primitive terrestrial reptiles and fully aquatic plesiosaurs. Judging by the proportions of the trunk, limbs and tail, nothosaurs, like modern crocodiles and marine iguanids, relied mainly on lateral undulation of the trunk and tail for aquatic propulsion. The rear limbs probably acted mainly as rudders. Differences in the structure of the pectoral girdles and forelimbs among nothosaurs and the elaboration of the coracoids suggest that the forelimbs were becoming important in propulsion, with emphasis on the back stroke. Pachypleurosaurus has the greatest degree of development of the anterior portion of the shoulder girdle, suggesting that the recovery stroke was of considerable importance. In this feature Pachypleurosaurus approaches the pattern seen in plesiosaurs. It is concluded that the transition between primitive terrestrial diapsids and plesiosaurs occurred via a succession of changes in patterns of aquatic locomotion. Early sauropterygians would have used almost exclusively the axial skeleton and its musculature in swimming. As in the majority of secondarily aquatic reptiles, stereotyped behaviour patterns of terrestrial locomotion would have rendered use of the forelimbs disadvantageous in the water, and they were progressively reduced to limit drag. The hind limbs were also reduced and initially functioned to control changes in depth and direction. The relations of the dermal and endochondral elements of the shoulder girdle in nothosaurs and plesiosaurs differ from those of the majority of early reptiles in having the scapulae superficial to the clavicles and the interclavicle superficial to the clavicles. This may be attributed to a redevelopment of the endochondral and dermal elements subsequent to a period of reduction associated with an early stage in aquatic adaptation. Nothosaurs and plesiosaurs are unique among secondarily aquatic diapsids in the emphasis on the forelimbs for propulsion. In plesiosaurs, both the fore and hind limbs are greatly enlarged and the significance of the tail for propulsion is much reduced. Both limbs may have served in a mode of swimming similar to that practised by the modern sea lions. Although individual vertebrae and bones of the limbs and girdles appear to show a transition from nothosaurs through primitive to advanced plesiosaurs, major differences in the proportions of the trunk, girdles and limbs clearly distinguish nothosaurs and plesiosaurs. This may be attributed to a marked reduction in the amount of lateral undulation of the trunk region and elaboration of the rear limbs as paddles in the immediate ancestors of plesiosaurs.