Chytridium olla A. Braun, the first described chytrid and an obligate algal parasite, is the type for the genus and thus the foundation of family Chytridiaceae, order Chytridiales, class Chytridiomycetes and phylum Chytridiomycota. Chytridium olla was isolated in coculture with its host, Oedogonium capilliforme. DNA was extracted from the coculture, and 18S, 28S and ITS1-5.8S-ITS2 rDNA were amplified with universal fungal primers. Free swimming zoospores and zoospores in mature sporangia were examined with electron microscopy. Molecular analyses placed C. olla in a clade in Chytridiales with isolates of Chytridium lagenaria and Phlyctochytrium planicorne. Ultrastructural analysis revealed C. olla to have a Group II-type zoospore, previously described for Chytridium lagenaria and Phlyctochytrium planicorne. On the basis of zoospore ultrastructure, family Chytridiaceae is emended to include the type of Chytridium and other species with a Group II-type zoospore, and the new family Chytriomycetaceae is delineated to include members of Chytridiales with a Group I-type zoospore.