Pupae of the tobacco budworm,Heliothis virescens(F.), diapausing and developing larvae of the codling moth,Laspeyresia pomonella(L.), diapausing larvae of the pink bollworm,Pectinophora gossypiella(Saunders), and diapausing pupae of an oak silk-worm,Antheraea pernyiGuérin-Ménéville, were exposed in cubicles to 1-h light breaks interjected into the scotophase following the photophase during fall, winter, spring, and early summer. The emergence of adult oak silkworms in the spring and early summer was accelerated compared with the controls by a 1-h light break interjected any time during the night. Termination of diapause in pink bollworm and codling moth larvae was delayed by a 1-h light break interjected into the dark period 20 h after dawn. Diapausing tobacco budworm pupae did not survive the winter but development to pharate adults occurred when 1-h light breaks were interjected 15, 16, 20, or 22 h after dawn.