Abstract The eye of the rhynchobdellid leech, Helobdella stagnalis, has been examined with the electron microscope. The eye is composed of a cup of pigment cells surrounding a compact mass of photoreceptor cells. In addition to pigment granules, the pigment-cell cytoplasm is characterized by mitochondria, a Golgi complex, and profiles of rough-surfaced endoplasmic reticulum. The photoreceptor cell contains a microvillous rhabdomere. The microvilli arise from the membrane of a large intracellular vesicle and obliterate much of its lumen. No connexion between the lumen of the intracellular vesicle and the extracellular space has been observed. The plasmalemma of the photoreceptor cell is folded to form thin pleats of cytoplasm which separate adjacent receptor cells from each other. No glial-like cells have been seen in the receptor cell mass. Directly subjacent to the microvilli and surrounding the intracellular vesicle is a tortuous and predominantly smooth-surfaced endoplasmic reticulum. A pair of centrioles is found near the rhabdomere. The cytoplasm around the nucleus is characterized by smooth- and rough-surfaced elements of endoplasmic reticulum, many mitochondria, and a Golgi complex. Proximally, the receptor cell narrows to form a nerve fibre which joins those from other cells to form the optic nerve.