1. 1. The proteins of the larval fat body of the southwestern corn borer, Diatraea grandiosella, were subjected to isoelectric focusing and disc electrophoresis in polyacrylamide gels to examine the diapause-associated protein (DAP). 2. 2. The fat bodies of last instar pre-diapausing larvae, as well as those of non-diapausing larvae which had been treated with juvenile hormone or a juvenile hormone mimic, were shown to store DAP. 3. 3. The protein fraction has an isoelectric point of 5.9, and appears to be a single polypeptide unit with a molecular weight of about 35,000. 4. 4. The protein was resistant to hydrolysis at 37°C by midgut enzymes and trypsin. 5. 5. De novo synthesis of DAP was demonstrated both in vitro and in vivo in the fat body of pre-diapausing larvae using radioactive leucine. 6. 6. Isolated fat bodies of 33-day old last instar pre-diapausing larvae incubated in macromolecule-free Grace's medium incorporated [ 3H]leucine selectively into DAP. 7. 7. Experiments employing [ 14C] and [ 3H]leucine and cycloheximide, an inhibitor of peptide bond formation, confirmed the synthesis of DAP in vivo. 8. 8. DAP was released unchanged from the fat body of mid-diapausing larvae in vitro. Since the protein does not accumulate in the haemolymph in vivo it may be structurally modified before or after release in vivo. 9. 9. The possible diapause-related functions of DAP are appraised.