Feed composition drives the viability of yellow mealworm (Tenebrio molitor Linnaeus, 1758) mass rearing, yet broad comparisons of feed materials remain scarce. In this research, the population growth of T. molitor was evaluated on 23 diets in a 65-day bioassay starting with 25 adults per replicate. The diets comprised 15 individual feed materials, including wheat bran as the control, and eight formulated diets with systematically varied protein and energy levels. At the end of the assay, all adults, larvae and pupae were counted, and population growth was expressed as the total number of individuals. Feed material significantly affected all population parameters (p ≤ 0.01), with the instantaneous rate of population increase (r_i) ranging from 0.0314 on dried distillers grains with solubles (DDGS) to 0.0603 on brewer’s yeast. Compared with the control (wheat bran), DDGS, flaxseed cake, brewer’s spent grain, barley flakes, oat flakes and breadcrumbs significantly reduced the expected population size. Brewer’s yeast, the highest-protein material (50.37%), produced the largest population, but it did not differ significantly from the control. A few pupae were recorded, but only on five of eight formulated mixtures. Because DDGS and flaxseed cake were also among the highest-protein materials, protein content alone did not predict population growth. Brewer’s yeast, rapeseed cake and sunflower meal did not differ significantly from wheat bran in either population size or larval biomass.