Mycosphaerella capsellae sp. nov. is reported as the teleomorph of Pseudocercosporella capsellae, the cause of white leaf spot on oilseed rape and other crucifers. Spermogonia and protoascomata were observed in white leaf spot lesions on oilseed rape pods in June 1990. Ascomata were mature by October and ascospores were dispersed in response to wetting by rain or dew. Ascomata were exhausted by early January 1991 and appeared not to over-winter. However, the teleomorph enables the fungus to survive the period between host crops, and the ascospores are the primary inoculum for infecting winter oilseed rape. Survival by means of stromatic knots, previously termed microsclerotia, is unlikely as they are not sclerotial in character but would appear to be primordia of spermogonia or ascomata. The teleomorph is described and compared with Mycosphaerella brassicicola which also occurs on oilseed rape.