Field experiments were conducted from 1989 to 1992 to determine the effects of Oebalus ornatus (Sailer) on Cica 8 rice, Oryza sativa L., yield and to study the population dynamics of the insect. The effect of O. ornatus was measured for seven population levels (0, 2, 3, 4, 8, 12, and 24 sexed pairs per 20 panicles) at three stages of grain development (flowering, milk, and soft dough stage). Insect feeding during the flowering and milk stages of grain development caused more damage than feeding during the soft dough stage. The action threshold was calculated to be 14 O. ornatus adults per square meter for the flowering and milk stages and 67 for the soft dough stage of grain maturity. Population densities that would reduce rice yield in southwestern Colombia were not observed during the 3 yr of the study.