The Erpobdella testacea cocoon membrane is studied for the first time by transmission and scanning electron microscopy. It has an ovoid form, displays a cambered dorsal side in which various micro-organisms are attached and a flattened ventral side. Symmetrically positioned, 2 opercula occur at the distal ends of the cocoon. The internal ultrastructure reveals fibrils (17.5nm) packed in layers forming C, S, bow shaped, parallel and stippling patterns lines, interrupted by Polygon-shaped cavities (1.8μm). Transverse sections show that each fibril presents an external dark part (6nm) and a central hole approximately 5.06nm in diameter. These features are discussed with bibliographic data signalized for other species. Phylogenetic and functional significations of the cocoon wall structure in leeches are suggested.