Phylogenetic maximum parsimony and Bayesian analyses of 60 collections belonging to 12 species of Albugo (Peronosporales) and two species of Pythium (Pythiales) were performed using nuclear large subunit ribosomal DNA sequences containing the D1 and D2 regions. These data were supplemented with detailed light and scanning electron microscopical analyses of oospore morphology, and the morphological data of insufficiently studied taxa (e.g. A. caryophyllacearum, A. gomphrenae) are revised. Molecular data revealed two main clades: one containing the collections from hosts belonging to the Caryophyllales and Asteraceae, and the other containing the collections from hosts belonging to the Brassicaceae and Convolvulaceae. Separation into these two clades was also corroborated by oospore morphology. Whereas the Albugo collections from Caryophyllales did not form a monophyletic lineage, the collections originating from Brassicaceae, Convolvulaceae and Asteraceae each formed highly supported monophyletic clades. According to DNA sequence data and oospore morphology, the host genus Amaranthus harbors two distinct species, Albugo amaranthi and Albugo bliti. The DNA sequence data further indicate that Albugo candida and Albugo tragopogonis each may consist of several distinct lineages, but additional data need to be collected before further taxonomic conclusions can be made.