Cochliomyia macellaria (Diptera: Calliphoridae) is known as the secondary screwworm because it causes secondary or facultative myiasis when the larvae feed on necrotic tissues. This fly has a significant medical and veterinary importance since it has been reported to transport eggs of Dermatobia hominis (human botfly), which can cause significant economic losses to livestock. Since this screwworm has been collected colonizing both pig carcasses and human cadavers, it is considered one of the most important species for forensic entomology studies. Scanning electron microscopy (SEM) gives detailed information on the morphological characteristics which can help identify the immature forms of the flies. The aim of this study was to describe and analyze the morphological characteristics of the eggs, all the larval instars, and the puparia of Cochliomyia macellaria using SEM. The egg is ellipsoid and the dorsal surface is concave. The islands inside the median area had no anastomosis, but some perforations could be observed. From the second larval instar onwards, besides the intersegmental spines, other bands of spines were observed at the abdominal segments. Two spiracular openings were visible on the first and second larval instars, which were not expected. These characteristics are specific to Cochliomyia genus. The number and the general aspect of the spine tips in the cephalic region, the intersegmental bands on the abdomen, and the number of the spiracular openings could together help identify C. macellaria.