Ultrastructure of the primitive epithelium of Echinostoma revolutum (Digenea: Echinostomatidae) cercaria was studied. The germinal balls and developing cercariae are covered with the primitive epithelium in daughter rediae. When the definitive tegument of the cercaria is differentiated, the primitive epithelium degenerates. The last remnants of the primitive epithelium in a cercaria can be detected at the stage when the lateral gland cells have released their secretion into the definitive tegument.