A disease associated with the infestation of cotton plants by the cotton or melon aphid, Aphis gossypii Glov. is described under the name of "vermelhão" or anthocyanosis. Symptoms of the disease resemble those resulting from magnesium deficiency. Aphids collected from diseased plants reproduced the disease when fed on cotton seedlings for 48 hours. Symptoms usually appeared from 12 to 30 days after inoculation, as chlorotic areas or spots that later turned reddish or purplish under strong light conditions. In plants kept in the greenhouse the chlorotic symptoms were more noticeable and very little or no reddening occurred. Reproduction of the disease was obtained with one aphid per plant, but transmission increased when five or 25 insects per plant were employed. The same species of aphid bred'on cucumber did not induce symptoms when fed on cotton seedlings. Reproduction of the disease was also obtained by grafting, the symptoms appearing in the rootstock. Perpetuation of symptoms has been obtained in the third vegetative propagation of a plant originally infected by aphids. The evidence outlined in the paper indicates that this cotton disease is probably caused by a virus and not due to an insect toxin. Alternative hypothesis are discussed.