Technical Abstract: We studied the ability of Tenebrio molitor L. (Coleoptera: Tenebrionidae) to self-select optimal ratios of two dietary components to approach nutritional balance and maximum fitness. Life table analysis was used to determine the fitness of T. molitor developing in diet mixtures comprised of four different ratios of wheat bran to dry potato flakes (9:1, 4:1, 7:3, and 3:2 = 10, 20, 30, and 40% potato, respectively). The four diet mixtures were compared to each other and a control diet of wheat bran only. In a separate experiment, relative consumption of the two diet components was determined among the four diet mixtures. Doubling time was significantly higher in groups consuming 9:1 and 4:1 diet mix ratios than the control and lower in groups feeding on 7:3 and 3:2 diet mix ratios. Consumption of individual diet components by T. molitor larvae deviated significantly from the expected ratios indicating non-random consumption. Mean percentages of dry potato consumed were 11.98, 19.16, 19.02, and 19.27% during the first 4wk and 11.89, 20.48, 24.67, and 25.97% during the next 3 wk period for diets with 10, 20, 30, and 40% potato, respectively. The self-selected ratios of the two diet components approached 4:1, which was the second best ratio for population growth. However, the shortest development time was observed at this ratio (4:1). Our findings fulfill the criteria for evidencing dietary self-selection behavior in T. molitor larvae. These findings may lead to new methods for optimizing dietary supplements for mass production of T. molitor.