Thallus development, zoospore size, zoospore ultrastructural complexity and organization, and flagellum length are cited as important in phylogeny of the Chytridiales (chytrids) and should be the bases for this classification. Discharge of zoospores is also cited as important although emphasis is not placed on operculation. It is proposed that simple, eucarpic, monocentric chytrids which discharge zoospores following dissolution of the sporangium wall evolved into multipapilliate species of Rhizophydium and 2 lines of evolution from these species are documented with examples. In one line they evolved into single-pored species such as R. pollinis-pini, then to Chytridium and tendochytrium-like chytrids and to polycentric chytrids such as Nowakowskiella and Cladochytrium. The second line evolved from multipapilliate Rhizophydium species to Phlyctochytrium and Entophlyctis, and to the order Blastocladiales. Rhizophlyctis rosea and some olpidiaceous chytrids, because of a fibrillar rhizoplast in their zoospores do not appear close to Rhizophydium-related chytrids and may require separate subordinal rank in classification. It is proposed that zoospores of the advanced members of the class Chytridiomycetes have reached limits for further zoospore development because of size; however, the zoospore is no longer essential for them because they have developed a filamentous habit of growth.
