As soon as a tentative diagnosis of facultative myiasis is rendered, the veterinarian must rule out the possibility of obligatory myiasis due to one of the primary screwworm maggots. In contrast to the facultative myiasis-producing flies, flies that indue primary mysiasis will not breed in carrion but lay their eggs at the edge of fresh, uncontaminated wounds of various warm-blooded animals. Within the wound, these larvae penetrate into, feed on, and rapidly devour fresh, live host tissue. The larvae of the primary screwworms basically require a living host. Because of the obligatory nature with regard to breeding in fresh wounds and the fear of introduction of screwworms from one continent to another, the practitioner should report infestations with primary screwworms in coutries where they are not autochthonous to the proper control authorities.