Screwworms, Cochliomyia hominivorax (Coquerel) (Diptera: Calliphoridae), were devastating pests in parts of North America and Central America before their eradication by the Sterile Insect Technique. Now a barrier is maintained to prevent re-entry of screwworms from endemic regions to those free of screwworms. Barrier maintenance requires mass rearing millions of competitive insects, sterilization by irradiation, and release into infested areas. Weight of the mature larvae (or pupae) is used for assessing fly quality for mass-reared screwworms. A method for producing flies of different sizes in the laboratory was developed. Small and large male flies were compared for sexual performance. A standardized measure of the wing length was used to characterize adult size. This measurement correlated positively with the larval and pupal weights. Although small males mated successfully, they mated with fewer females than did large males when each was placed alone with a group of females. When placed together with large males, small screwworm males were out-competed (41.2% as competitive) by the large males. Although behavior of male screwworm flies in laboratory conditions has not been demonstrated to be similar under field conditions, knowledge of mating competitiveness of sterile males of different sizes is valuable to the screwworm eradication program.