To reveal phylogenetic relationships within the Peronosporomycetes (Oomycetes), we sequenced a part of the nuclear rDNA coding for the ribosomal large subunit of 46 Peronosporomycetes species and one representative of the Xanthophyta. The main emphasis of our study was put on the phylogenetic relationships within the Saprolegniomycetidae. We supplemented our data with a sequence of Phytophthora megasperma Drechsler from GenBank. Two sets of sequences were analysed using the neighbor-joining

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... od, statistically supported by the bootstrap method, as well as the maximum parsimony method. Our results are well compatible with the tripartite subclassification of the Peronosporomycetes into Saprolegniomycetidae, Rhipidiomycetidae and Peronosporomycetidae, as well as with the placement of the orders Saprolegniales and Leptomitales in the Saprolegniomycetidae. Pachymetra chaunorhiza Croft & Dick, which has been placed in the Sclerosporales, was grouped within the Saprolegniales. Within the Peronosporomycetidae, the orders Peronosporales and Pythiales could not be separated. There are indications that Phytophthora de Bary and the Peronosporales form a common natural group. The genus Achlya Nees proved to be a heterogeneous group.