The present research reassesses the taxonomic position of species of Leptosphaeria described from plants in the family Rosaceae. In doing so, this wori< emphasizes a concept of Leptosphaeria based on its type species. Because discrete groups of morphologically similar species were found on related hosts (Nixlulosphaeria on Compositae, Phaeosphaeria on Gramineae and Entodesmiitm on Leguminosae), a goal of the study was to determine whether a distinct group of related species could be found on Rosaceous hosts. Also, approaching the taxonomy of this large, complex genus on the basis of host family is a convenient way of dividing the large numbers of Leptosphaeria species into smaller, workable groups. Fifty-five species described from Rosaceous hosts have been included in Leptosphaeria (see appendix). Efforts were made to locate and obtain type material for all of the described species. The names of herbaria providing material are abbreviated in this dissertation according to Index Herbariorum (Holmgren et al. 1990). Whenpossible, type specimens were studied, supplemented by other collections, and the species were redescribed and illustrated from the type material. Unfortunately, type material could not be located for all of the species; in such cases, descriptions were based on other material only when the resulting concept of the species was obviously in accordance with the original description. From this work, it became apparent that no discrete group of related species is specialized on the Rosaceae. Thespecies studied were determined to represent a mixture of Loculoascomycetes and Hymenoascomycetes, and non-Leptosphaeria species were reassigned to more appropriate genera.