Interactions of the parasitoid Lysiphlebus testaceipes (Cresson) and the greenbug, Schizaphis graminum (Rondani), on greenbug-resistant ÔCargill 607EÕ (antibiosis), ÔCargill 797Õ (primarily tolerance), and -susceptible ÔGolden Harvest 510BÕ sorghum, Sorghum bicolor (L.) Moench, were tested using three levels of biotype I greenbug infestation. The parasitoid infestation rate was 0.5 female and 1.0 male L. testaceipes per plant. For all three greenbug infestation levels, the parasitoid brought the greenbug under control (i.e., prevented the greenbugs from killing the plants) on both resistant hybrids, but it did not prevent heavy leaf damage at the higher greenbug infestation rates. At the low greenbug infestation rate (50 greenbugs per resistant plant when parasitoids were introduced), greenbugs damaged 5 and 18% of the total leaf area on ÔCargill 797Õ and ÔCargill 607EÕ, respectively, before greenbugs were eliminated. Leaf damage was higher for the intermediate infestation study (120 greenbugs per plant), 21% and 30% leaf area were damaged on the resistant sorghum hybrids ÔCargill 797Õ and ÔCargill 607EÕ, respectively. At the high greenbug infestation rate (300 greenbugs per plant), heavy damage occurred: 61% on ÔCargill 607EÕ and 75% on ÔCargill 797Õ. The parasitoids did not control greenbugs on the susceptible sorghum hybrid ÔGolden Harvest 510BÕ. L. testaceipes provided comparable control on both greenbug-resistant hybrids. This study supports previous studies indicating that L. testaceipes is effective in controlling greenbugs on sorghum with antibiosis resistance to greenbugs. Furthermore, new information is provided indicating that L. testaceipes is also effective in controlling greenbugs on a greenbug-tolerant hybrid.KEY WORDS parasitoid, resistance, biological control, antibiosis and tolerant sorghum hybrids THE PARASITOID Lysiphlebus testaceipes (Cresson) has been identiÞed as one of the most important natural enemies of the greenbug, Schizaphis graminum (Rondani), in the United States (Hunter 1909, Webster and Phillips 1912, Wadley 1931, Sekhar 1957, Schlinger and Hall 1960, Knipling and Gilmore 1971, Hight et al. 1972. L. testaceipes is often men