Many Siberian elms (Ulmus pumila L.) were diagnosed as having Dutch elm disease in St. Paul, Minnesota, in 1982. Because of the reported resistance of Siberian elms to Dutch elm disease, Siberian elms and American elms (U. americana L.) were inoculated with aggressive isolates of Ceratocystis ulmi (Buism.) C. Moreau obtained from each host to determine if a more virulent pathogen were present. Sapling-sized American elms (3–15 cm diam) inoculated in the field developed symptoms faster after inoculation with Siberian elm isolates than after inoculation with American elm isolates. Both isolates, however, killed American elms within 3 mo. Two months after inoculation, only isolates from Siberian elm caused symptoms in Siberian elm, a mottling of the foliage on inoculated branches. Siberian elm isolates caused more staining and moved greater distances proximally in the xylem of Siberian elm branches than did American elm isolates. Results suggest the existence of a C. ulmi strain more virulent to Siberian elms in Minnesota. References: Gibbs, J. M., and Brasier, C. M. Nature 241:381, 1973.