The fine structure of a small phagotrophic protist, Diplonema ambulator, associated with deteriorating leaves of some species of the common fresh-water aquarium plant, Cryptocoryne is described. The organisms bear two short flagella which arise subapically and have a flexible cell surface often exhibiting pronounced changes in shape. Under the plasma membrane is a single layer of interconnected microtubules. Adjacent to the microtubules is a peripheral reticulate mitochondrion with plate-like cristae and numerous dense beaded inclusions. A large complex microtubular-based feeding apparatus extending to the cell surface is surrounded by several food vacuoles. Because the organism fits the light microscopic description of the genus Diplonema and has ultrastructural features attributable to Isonema we recommend that these taxa be regarded as synonymous and that existing species of Isonema be transferred to the genus Diplonema.