New definitions of the form-genera Phoma Sacc. and Ascochyta Lib., based on developmental criteria, are presented. Phoma species show phialidic ontogeny. The first conidium is produced within a papillate protrusion of the undifferentiated parent cell; after a conidium secedes the basal part of the papilla remains as a collarette on the conidiogenous cell which may show two or three layers corresponding with the layers of the original papilla. The conidia secede by a three-layered septum and are in principle one-celled, although secondary septation may occur, especially in vivo (0-95%). In vitro under normal laboratory conditions the majority of conidia however always remain one-celled. The pycnidia are usually glabrous but also may be hairy or setose. Ascochyta species show annellidic ontogeny. The conidia arise as relatively thin-walled protrusions from undifferentiated parent cells. The secession of successive conidial primordia by a three-layered septum however may occur at approximately the same level, resulting in an increasing collar of periclinal annellations, which under the light microscope looks like the collarette of a phialide. After or incidentally also before secession the conidia become two(or more-) celled by invaginations of a newly produced inner wall layer (distoseptation). In this genus therefore conidial septation is an essential part of the conidium completion, which explains that in vivo as well as in vitro the conidia are always mainly two- (or more-) celled. The pycnidia are glabrous or sometimes hairy. These new generic concepts imply that most of the species usually placed in Ascochyta viz. those in Ascochyta sect. Phyllostictoides Zherbele, belong to Phoma as are many species at present placed in Phyllosticta, Diplodina and Pyrenochaeta. Pyrenochaeta mali Smith is shown to be identical with Phoma herbarum. For Pyrenochaeta acicola (Lév.) Sacc. the new name Phoma leveillei is introduced.