Biotrophic Peronosporomycetes (formerly Oomycetes) have been a challenge for taxonomists since their dis covery. Not only do they share with other microorgan isms a low degree of morphological variability, they even hide these characters from observation by completing most of their life history inside their host plants. Only their reproductive structures on the plant surface and disease symptoms on the host plant reveal their presence and enable scientists to collect and investigate these organisms. It, therefore, is not surprising that the internal and external phylogenetic relationships of the Peronosporomycetes have recurrently been discussed unequivocally, and have faced considerable regrouping within the past decades, as new tools have become available for studies of plant systematics. Thus bio chemical properties such as the glucan cell wall and physiological differences in the biosynthesis of lysine were used early to separate Peronosporomycetes from eumycotic fungi. The Peronosporomycetes had been formerly placed in the Eumycetes due to superficial similarities in hyphal organization and osmotrophic nutrition. Fundamental differences in the sexual repro duction and karyology of Peronosporomycetes and Eumycetes (reviewed by Tommerup 1981) supported this separation and hinted that a common ancestor of the Peronosporomycetes and chromistan algae (another polyphyletic assemblage that underwent dramatic regrouping with the discovery of secondary endosym biosis) should be sought. The type of flagellum (anteri orly directed and equipped with tripartite hairs) was finally recognized as the decisive synapomorphism that Peronosporomycetes share with other straminipilous groups (Patterson 1989; for formal diagnosis, see Dick