Pathogenicity of Xylella fastidiosa was demonstrated in American elm seedlings. Ten 4-mo-old seedlings were stem-inoculated with a pure culture of X. fastidiosa obtained from a naturally infected elm. One year after inoculation, all inoculated seedlings developed leaf scorch symptoms characteristic of naturally infected trees. X. fastidiosa was isolated from six inoculated seedlings but not from 10 symptomless control seedlings. Isolates of X. fastidiosa obtained from a naturally infected elm and a previously inoculated sycamore were pathogenic in seedlings of elm and sycamore, respectively, but not in reciprocal hosts.