A subdivision of the *Eumycota
consisting predominantly of aquatic fungi or fungi that flourish under particularly damp conditions. It contains about 190 genera and some 1280 species. Its members characteristically produce motile cells and are thus called zoosporic fungi. The number and type of flagella possessed by the zoospores is used to divide the Mastigomycotina into three classes: the *Chytridiomycetes
, whose zoospores have one posterior whiplash flagellum; the Hyphochytridiomycetes, whose zoospores have one anterior flimmer flagellum; and the *Oomycetes
, whose zoospores have an anterior flimmer flagellum and a posterior whiplash flagellum. The diversity of zoospore structure implies the Mastigomycotina are polyphyletic and hence not a natural group.