Abstract. Phreatophasma aenigmaticum is a mysterious tetrapod from the earliest middle Permian of Russia, represented by a single femur. At various times since its original description it has been considered a therapsid synapsid, a pelycosaurian-grade synapsid from the family Caseidae, and most recently a seymouriamorph amphibian. Using up-to-date knowledge of the postcranial morphology and evolution of early synapsids, the specimen is re-evaluated and subjected to cladistic analysis. Seymouriamorph and therapsid affinities are rejected, and a caseid affinity is supported based on the deep intertrochanteric fossa; the widely spaced distal condyles; the short, robust femoral shaft; and the lack of a longitudinal ridge enclosing the posterior margin of the intertrochanteric fossa. When included in two cladistic matrices, the first a global analysis of basal synapsids and the second devoted to caseids, Phreatophasma is found to occupy a basal position within caseids, retaining plesiomorphic characters such as the lack of compression of the anterior condyle of the femur and the almost identical distal extent of the two condyles. The recognition of Phreatophasma as a basal caseid has great implications for the evolution and biogeography of this family. This is only the second example of a caseid from the palaeo-temperate region of Russia, and it is not closely related to the first (Ennatosaurus tecton), implying at least two distinct dispersal events from the palaeoequatorial to temperate latitudes. It also implies that a number of plesiomorphic characteristics of caseids, including small body size and a relatively long femur, were retained as late as the middle Permian, a time when caseids were otherwise represented by large herbivorous taxa.