When female screwworm flies,Cochliomyia hominivorax(Coquerel), were allowed continuous opportunity to oviposit on artificial medium, more than 32% of a population of wild-type (wound-reared F1–F5. progeny of native flies) females failed to oviposit, but only 3% of a population of laboratory-adapted females (progeny of strains reared on artificial medium>150 generations) laid less than 1 egg mass. Also, peak oviposition of the 1st egg mass occurred about 4 days earlier with the laboratoryadapted females than with the wild-type females, and there was a highly significant difference (P<0.01) between the 2 groups in the mean number of egg masses per female. Oviposition by virgin wild-type females occurred rarely (about 4%), but there were almost no virgin females among laboratory-adapted populations by the time the flies reached sexual maturity.