1. Gnomonia ulmea (Schw.) Thum., the cause of the most common elm leaf spot in America, has been reported as occurring on five of the six native species of elm in this country and is of wide distribution, being found throughout the entire range of its hosts. Its normal host, on which it is most commonly found, is Ulmus americana. The fungus is not ordinarily of much economic importance, but may cause considerable injury to seedlings and young trees in nurseries by producing premature defoliation. 2. Unlike most of the Ascomycetes, the perithecial stage of the fungus begins its development in the living leaf early in the spring. The young perithecium develops in the palisade tissue beneath a subcuticular black stroma. 3. An ascogonium is found in the young perithecium, but there is no trichogyne. 4. An interascicular pseudoparenchyma is found present in the perithecium almost until the period of maturity. 5. In the process of ascospore expulsion an entire ascus enters the lower part of the ostiolar canal, ...