The fine structure is described of a new nonphotosynthctic flagellate, named Desmomonas prorhynchi gen. et sp. n„ cndoparasitic in the lecilhoepilheliate turbellarian Prorhynchus sp. The flagellates surround cell masses enveloped by a protective wall, to which adhere many small, regularly arranged cylindroid bodies; these structures are reminiscent of epicuticular projections characterising the body walls of some oligochaetes and polychaetes. The cells of a cell mass arc joined to the enveloping wall by spot hemidesmosomes. Their cytoplasm contains Golgi systems, mitochondria, fibrillar dense bodies, paired centrioles, and rod-shaped bacteria. Certain cells possess four ccntrioles. Ü. prorhynchi bears two flagella, and is attached temporarily to a cell mass by an anterior process ending at a highly organised desmosome. The flagellate plasma membrane has a fuzzy coating. Hair-like plasmalcmmal extensions project from the base of the anterior process. A mitochondrial derivative containing dense spheres lies some distance from the flagellar basal bodies. There is no Golgi apparatus. Other cytoplasmic constituents of the flagellate morphology are vacuoles, and microneme-like bodies. The two flagella, similar and without paraxial rods, arise from a flagellar pocket; neither is recurrent. The new protist cannot at present be assigned to any existing zoomastigophorean order.