AbstractA pathological condition occurring in the lesser octopus, Eledone cirrhosa, is described. This results in progressive ulceration of the skin of the animal, followed by oedema of body tissues and death. Light and electron microscope studies of the skin ulcers reveal the constant presence of thraustochytrid-like organism(s) and often, although not in all instances, the presence of a presumed labyrinthulid. Studies are currently in progress to identify both these organisms and to establish the role that they play in the condition described.