We describe a virtually complete, articulated skeleton of the pantolestid Palaeosinopa from the late Wasatchian of the Green River Formation, Wyoming. It is the oldest known pantolestid skeleton and the most complete pantolestid remains known from North America. We also illustrate and compare a second nearly complete skeleton of Palaeosinopa from the same locality in the Fossil Butte Member. Both skeletons are dentally adult (with fully erupted third molars and permanent premolars) but show unfused and in some cases dislodged epiphyses of the long bones and caudal vertebral centra. The skeletons come from Thompson Ranch Quarry in the lake-margin facies of Fossil Lake. Based on the intact, articulated condition of the principal skeleton, it was apparently preserved in a locally anaerobic environment; disruption of the pelvic and proximal caudal region probably resulted from explosion of gases accumulating inside the carcass. The abdominal region of this specimen contains fish remains. In addition to the Fossil Butte skeletons, other pantolestid postcranial remains from North America are described. Palaeosinopa closely resembles middle Eocene Buxolestes from Messel, Germany, in most respects, except for having a longer tail. It was medium-sized, about 1 m long from snout to tip of tail, and weighed about 5 kg. The skull features a pronounced nuchal crest, and the spinous processes of C2 and the proximal thoracic vertebrae are strongly developed. The long tail contains 30 vertebrae, with very wide transverse processes and haemal arches proximally, and double transverse processes well developed from Ca 9 to Ca 12. The sternum and clavicles are robust, and the scapula has well-developed acromion and metacromion processes. The humerus is short, robust, and sigmoid (weakly S-shaped) in lateral view, with prominent pectoral, deltoid, and supinator crests. The olecranon is about one-third of ulnar length, and the radial head is transversely wide. The manus is pentadactyl and generalized, with moderately stout metapodials and phalanges, including long, shallow, slightly curved ungual phalanges similar to those of digging and some semiaquatic mammals. The hind limb is longer than the forelimb and of relatively generalized anatomy. The ilium is somewhat longer than the ischium. The femoral shaft is anteroposteriorly flattened in pantolestids, and the distal end of various specimens resembles that of either Lutra or Castor. The tibia is slightly sinuous in lateral profile and in some specimens is distally fused with the fibula. The calcaneus is distally short and characterized by a large tuber, prominent plantar tubercle, and a large, distal peroneal tubercle. The pes is longer than the manus and has slightly wider unguals. The anatomy and taphonomy of the Fossil Butte skeletons, strengthened by supplemental specimens, suggest that pantolestids were both proficient burrowers and capable swimmers, as previously inferred from the analysis of the Messel specimen of Buxolestes. When swimming, they probably propelled themselves by hind-limb paddling and dorsoventral undulation of the tail, in a manner approximating that of otters.