AbstractPlasmodiophorids are a monophyletic group with uncertain systematic affinities. Features of the group include cruciform nuclear division; obligate, intracellular parasitism; biflagellated, heterocont zoospores; and environmentally resistant resting spores. Economically significant members of the group includePlasmodiophora brassicae, the causative agent of clubroot of cabbage;Spongospora subterranea, the causative agent of powdery scab of potato; and two members of the genusPolymyxa, vectors for several plant pathogenic viruses.