Each author contributes a separate letter on the possible nature of certain cytoplasmic particles of the mycetocytes of aphids. Trager thinks these particles may be micro-organisms, whereas Lanham defends his former opinion that they are more probably mitochondria. Mycetocytes are intimately associated with the fat bodies, and it is tentatively postulated that they are centers of Krebs cycle activity, thus providing the energy for the synthesis of fats, proteins, and glycogen, these then being stored by the fat cells. DNA has not been demonstrated in the cytoplasmic particles of the mycetocytes.