A new species, Rhexoxylon piatnitzkyi is described and illustrated. It is a small stem showing the primary organization of the vascular cylinder and has enabled the authors to present a new interpretation of the development of the complex mature stem structure of Rhexoxylon. Together with further specimens of larger stems described provisionally as Rhexoxylon sp.A the material has revealed new anatomical details. The diagnosis of Rhexoxylon and of R. tetrapteridoides Walton are emended. R. waltonii Krausel is reduced to a synonym of R. africanum Bancroft. R. priestleyi (Seward) Walton is restored to Antarcticoxylon Seward because of differences judged to be of generic value. The affinities of Rhexoxylon are discussed. The present record from Triassic strata in north-west Argentina is the first occurrence of Rhexoxylon outside Africa and supports a stratigraphical correlation of the beds (Ischigualasto Formation) with the Stormberg Series of South Africa.