The ultrastructure of active and inactive corpora allata from last instar larvae of the southwestern corn borer, Diatraea grandiosella, was examined. Active glands were obtained from pre-, early, and mid-diapausing larvae; inactive ones from late and non-diapausing larvae. Each gland contains 13 to 18 cells which have the following common features: well developed smooth endoplasmic reticulum, scattered rough endoplasmic reticulum and free ribosomes, microtubules, vacuolated nucleoli, and interlocking plasma membranes. The gland contains intercellular deposits, and is supplied by regular and neurosecretory axons.