Chaetopterus variopedatus (Renier) is a tube-dwelling worm that has note worthy luminescent powers. Production of light is by an extracellular process resulting from the discharge of a luminescent slime into the sea water. Because of its luminescent ability, the epidermal glands of Chaetopterus have been repeatedly described in detail, but it is still not clear what elements produce the luminescent secretion. I have, therefore, re-examined the histology of the light-producing regions of Chaetopterus with a view to determining the character of the photocytes or light-producing glandular cells.