A total of 37 cases of myiasis in the cotton-tail, the jack rabbit, the opossum, and the deer harbored natural infestations of the screwworm, Cochliomyia americana. In one instance 570 flies emerged from the collection of 3d instar larvae from a jack rabbit and an estimate of 3000 larvae were present in a deer. 4% of the rabbits may carry infestations of C. americana. The predisposing causes which render animals subject to fly attack are rabbit bots, gunshot wounds, boils, disease, birth of young, injury by predators, fighting, and brush and wire cuts. The fact that wild life serves as a considerable source of infestation of the screwworm makes more difficult the control in livestock.