Insect injury-crop yield change functions (i.e., regression equations) and economic injury levels (ElLs) were developed and compared for the bollworm Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) and the tobacco budworm, Heliothis virescens (F.) (Lepidoptera: Noctuidae) on cotton, Gossypium hirsutum (L.). Functions and ElLs were developed for the varieties Deltapine-50 and TAMCOT CD3H at growth stages of one-third grown flower bud, second week after initiation of anthesis (flowering), and 30 days after initiation of anthesis. Significantly different functions were observed between H. zea and H. virescens, varieties, and growth stages. EILs ranged from 4 to 9 percent injured reproductive organs at one-third grown flower bud and 3 to 7 percent at second week after initiation of anthesis. Differences in yields were not observed for any infestation level at 30 days after initiation of anthesis. A maximum difference of four percent injured reproductive organs, i.e., flower buds and capsules, were observed in ElLs between insects. More accurate EILs were calculated using regression analysis compared to analysis of variance. ElLs determined from injury caused by the fewest neonates resulted in yields significantly different from yields of uninfested plots (from analysis of variance) ranged from 12 to 32 percent injured reproductive organs.