The selection conducted at L'Assomption, P.Q., from 1950 to 1963, has confirmed the very high susceptibility of the white elm (Ulmus americana L.) to the attack of Ceratocystis ulmi (Buis.) C. Moreau and the absence of resistant strains in nature. Thermal neutrons or X-ray treatment of seeds has not appreciably modified seedlings reaction to infection, except in 4 cases out of almost 150,000 in which no symptoms were apparent before the fifth annual inoculation or more. One individual has resisted nine consecutive tests and its cuttings did not take the infection after two assays. Resulting from a mutation produced by X-rays, this vigorous clone has a normal foliage but is of a smaller size than other elms of the same age. This case of immunity in the white elm is related to that of the Asiatic species and certain European strains also of slow-growing habit and resistant.