Larvae of the yellow mealworm, Tenebrio molitor L., have been used to evaluate nutritional quality of proteins and protein isolates. However, such investigations have been complicated by lack of knowledge of dietary requirements of the larvae. To determine essential dietary amino acids for growth of Tenebrio molitor, single amino acids were deleted from the amino acid mixture of the diet. Diets were maintained isonitrogenous with supplementary glycine and, in the case of deleted glycine, with glutamic acid. Growth, as measured by gain in weight, and survival were observed over a 4-week period at 27 plus or minus 0.25 degrees and 65 plus or minus 5% relative humidity. The results indicate that larvae of Tenebrio molitor require a dietary source of the same 10 amino acids essential for growth in rats, other vertebrates, and some protozoa. They also showed that serine, tyrosine, glutamic acid, and possibly glycine were dispensable for growth in this insect. Alanine, cystine, proline, and aspartic acid appeared semidispensable. Survival over the 4-week experimental period was unaffected by deleting amino acids from the diet. The results are discussed in relation to amino acid requirements of other insects and to suggested improvement of the diet of the present investigation.