A selection experiment with Tribolium castaneum was carried out to investigate the effect of the intensity of selection at the first stage on the efficiency of two-stage relative to one-stage index selection for a given final intensity of selection (20%). Four lines-a one-stage index line (I1) and three two-stage index lines (I21, I22, I23)- were involved in the experiment; selection over four generations was based on an index that incorporates pupal weight and number of eggs laid. Selection took place in a single operation after both traits were measured in line I1. Pupal weight was used in the first stage, and both traits were used in the second stage in lines I21, I22, and I23, with a decrease in proportion selected at the first stage (75%, 50%, and 25%, respectively). There was substantial improvement in aggregate breeding value (pupal weight plus egg laying) and in both component traits in all lines; the response obtained in aggregate genotype and egg laying was significantly smaller in line I23. Expected and observed responses agreed for the aggregate genotype and egg laying. This study illustrates the effectiveness of two-stage index selection, because culling as much as 50% in the first stage did not significantly decrease the observed response in the aggregate genotype. © 1991 The American Genetic Association.