The density relationship of Schizaphis graminum (Rondani) on resistant and susceptible grain sorghum and the interaction of Lysiphlebus testaceipes (Cresson) was studied in field cages. A simulated early natural infestation of 450 female parasitoids/acre was compared to a simulated early release of an additional 2500 parasitoids/acre. The populations of greenbugs in all cages increased for 4 wk after infestation regardless of the presence of parasitoids or plant resistance, but the increase was much more pronounced on susceptible plants. Resistant plants had about the same pattern of greenbug dynamics regardless of the level of parasitoids; in contrast, susceptible plants treated with the high level of parasitoids had less than 1/5 the peak number of greenbugs found when low levels of parasitoids were present, and the decline in the greenbug population started ca. 1 wk earlier. The numbers of mummies on resistant plants were similar for treatments with low and high levels of parasitoids. However, treatment of susceptible plants with the high level of parasitoids in comparison with the low produced a peak occurrence of mummies ca. 1 wk earlier. Plant data at harvest supported the counts of the pest and parasitoid. In general, the results verified theoretical calculations by other workers, but they also demonstrated that plant resistance was more promising than the release of L. testaceipes as an alternative to insecticidal control of greenbugs on sorghum.