An outline is presented for the reclassification of the Chytridiales into two orders based on fundamental differences in zoospore ultrastructure. The new order Spizellomycetales includes species with the Phlyctochytrium–Entophlyctis type of zoospore and also Karlingia, Olpidium, and Rozella. Four new genera, Spizellomyces, Gaertneriomyces, Triparticalcar, and Kochiomyces replace the Phlyctochytrium complex. The revised Chytridiales includes species with the Rhizophydium–Chytridium–Nowakowskiella type of zoospore, as well as Synchytrium,

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Developmental stages and morphological variation are described for three species of fungi from the rumen of a Holstein steer. Neocallimastix sp. is monocentric and has endogenous, rarely exogenous, sporangial development. Piromyces communis is monocentric, rarely polycentric, and has both endogenous sporangium development and exogenous sporangia on sporangiophores. Orpinomyces bovis is described for a polycentric fungus with terminal and intercalary sporangiophore complexes that develop from the rhizomycelium, and polyflagellate zoospores. The advantages and limitations of the light microscope and the enigmatic problem of classification of diverse and polymorphic rumen fungi are discussed.

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