The taxon Oomycota (Oomycetes) contains some very important pathogens of plants and animals. Perhaps the most infamous of these belong to the family Pythiaceae, and particularly to the genera Pythium and Phytophthora, many of which are extremely destructive pathogens of plants. The Oomycetes are “fungi” in the sense that they are heterotrophic, eukaryotic organisms which obtain their food by absorption, that they possess a mycelial stage, and that they produce spores by asexual and sexual means. The precise relationship of the Oomycetes to other fungi and to the heterokont algae, which they resemble in many respects, has been debated for over a century. The Oomycetes appear to be quite distinct both from the higher fungi and from the other Mastigomycetes. The sterol metabolism of the Oomycetes also differs dramatically from that of the higher fungi. Sequences for gene coding for certain proteins from Oomycetes also reflect the extent of the evolutionary separation between the Oomycetes and the higher fungi.