A naturally occurring triploid elm hybrid was found in the American elm,UlmusamericanaL., planting on the National Mall in Washington, D.C. Chromosome examinations of mitosis in root tips and meiosis in pollen mother cells showed a chromosome complement of 2n = 3x = 42. The chromosome alignment at meiotic metaphase I was predominantly 14 bivalents and 14 univalents, indicating that one parent was an American elm, which contributed the bivalents through autosyndetic pairing. When DNA underwent restriction digestion withHindIII and probed with a 9.5-kilobase cloned ribosomal DNA fragment from pea,PisumsativumL., the triploid hybrid showed a band not found in American elms. Seed from the open-pollinated parent tree had low viability and seedlings were highly variable in height, leaf size, and shape. The small leaves of some seedlings suggest that the other parent was a species with leaves smaller than American elm. The hybrid was found to be resistant toOphiostomaulmi(Buis.) Nannf. When twig crotches were inoculated, only 14% developed wilt compared with 63% in the American elm. None of the 22 hybrid trees inoculated developed systemic wilt compared with 8 of the 18 American elms inoculated. The hybrid, which is easily propagated by softwood cuttings, has rapid growth, good crown structure, and many characteristics of the American elm.