Order: Ameboflagellates with 2 heterodynamic acronematic flagella (Illustrated Guide, 1985).
Genus: Biflagellate, both arising from anterior end; one directed anteriorly and the other runs backward over body surface as a trailing (recurrent) flagellum; body plastic (Kudo, 1966). Small (10-15 μm); body is generally pyriform to cylindrical except for an ameboid variable posterior (Illustrated Guide, 1985).

C. crassicauda 3 clips 20 μm Kurome river Saitama, 1999	C. crassicauda 4 clips dividing cell Kurome river Saitama, 1999	Cercomonas sp. 10 clips μm xxxx xxxx, 1999
Cercomonas sp. 1 clip μm Kurome river Saitama, 1999	Cercomonas sp. ? 1 clip μm Higashi-hiroshima Hiroshima, 1998	Cercomonas sp. 3 clips μm Uruguay, 1999
Cercomonas sp. 1 clip μm Fujimi Saitama, 2000