
A fungus was always found to be associated with the necrotic patches of the corals in the Gulf of Mannar Biosphere Reserve. It was nonsporulating under both natural and culture conditions. Hyphae of the fungus were septate, highly melanised and presented cottony appearance in the colony. Lethal potency of the acidified ethanol extract of this fungus was tested by the mouse bioassay method and the LD50 is determined as 75mg per mouse.