The normal pupation period of Tribolium confusum on whole wheat diet averages 18 days. In presence of a purified diet containing 19 crystalline amino acids known to exist in casein, the pupation period is increased to 28 days. Removal of one of the amino acids considered as nonessential for growth in the rat from such a diet, results in pupation periods falling between 26 and 32 days. On the other hand, omission of lysine, threonine, phenylalanine, methionine, isoleucine, and arginine, one at a time, increases the pupation period from 61 to 70 days. If valine, tryptophane, histidine, or leucine are left out singly, pupation is prevented, though two-thirds of the larvae survive. It is concluded that the amino acids known to be essential for growth in the rat are also essential for growth and pupation of the larvae of T. confusum. Four of these essential amino acids appear to be connected with the phenomenon of pupation.