Two elm taxa occur naturally in the Iberian Peninsula: the Field elm (Ulmus minor) and the Wych elm (U. glabra). In addition, a third taxon, the foreign Siberian elm (U. pumila), was probably introduced in the 16th century as an ornamental tree and has spread spontaneously throughout the Peninsula. The natural hybridization between U. minor and U. pumila produced new individuals whose morphological traits appear to be mixed. Ulmus pumila, as well as its hybrids, has a high resistance to Dutch elm disease (DED). For this reason, it is commonly used in breeding programmes. Extensive hybridization and the high mortality produced by the last DED epidemic have endangered the conservation of the native elm. In this study, isozyme analyses are used to characterize the taxa U. minor and U. pumila. Siberian elms from Spain and China are compared with the native U. minor. Siberian elm produces isozyme patterns that completely differentiate it from U. minor. Three loci are completely different between the species: 6Pgd2, Mdh1 and Prx2. Isozyme markers can also be used to distinguish native elms from the hybrids that have evolved for generations.