Technical Abstract: Mitochondrially encoded cox2 sequences were used to infer evolutionary relationships of downy mildew and white rust taxa in a data set of 36 peronosporomycete isolates. The data set of 599 aligned nucleotides was analyzed using neighbor joining, maximum parsimony, and maximum likelihood. These phylogenetic analyses did not infer separate monophyletic orders for the Pythiales versus downy mildews (Peronosporales), but do indicate that separation of monocot and dicot-infecting downy mildews into separate subclasses is not justified. Analyses of three species of Albugo, however, infer that Peronosporales are a polyphyletic group, unless this order is expanded to include species of the Pythiales and Rhipidiales. Whereas all examined downy mildew cox2 amino acid sequences bore the signature indel LEF/Y characteristic of the subclass Peronosporomycetidae the three Albugo species did not. Instead, the LEF/Y signature indel was replaced by a highly variable indel unique to each Albugo species. Collectively, these results indicate that the white rusts are only distantly related to downy mildews and constitute a distinct order basal to other orders within the Peronosporomycetidae.