An internal transcribed spacer-based phylogenetic analysis is presented for 50 described taxa, Phytophthora, a diverse group of Pythium species and for representatives of the Saprolegniales and Peronosporales. The main lineages within Phytophthora were compared with the species groups defined by traditional taxonomy. This was approached by a study of the rDNA internal transcribed spacer regions of >234 isolates. Evolution of breeding systems and ecological strategies within Phytophthora were examined by comparing internal transcribed spacer phylogeny with biological characteristics. The position of Phytophthora in relation to other oomycete genera, particularly Pythium and Peronospora was also investigated.