The ultrastructure of a free-living retortamonad, Chilomastix cuspidata, originally described as Percolomonas cuspidata Larsen and Patterson, is reported for the first time. This species has a cosmopolitan distribution and has been reported from marine, brackish and freshwater sites with low levels of oxygen. There are four flagella which are located subapically at the head of a ventral groove. The basal bodies give rise to five roots: two major microtubular roots which support the walls of the ventral groove, two minor microtubular roots and one striated root. The root lying in the right wall of the groove also supports the cytopharyngeal region. An electron-dense ''lapel'' extends dorsally around the site of flagellar insertion and gives rise to microtubules which support the cell membrane. No mitochondria nor dictyosomes were observed, On the basis of this information, the organism is assigned to Chilomastix, These observations are also discussed in relation to the origin of mitochondria within eukaryotes.
