Sperm transfer and fertility of the gypsy moth, Lymantria dispar (L.), held as pupae under constant temperature and 24-h light, 24-h darkness, or a photoperiod of 16:8 (L:D) h were compared with males held in cyclic temperature and 24-h light, 24-h darkness, or a photoperiod of 16:8 (L:D) h. Constant temperature and constant light or darkness severely reduced its ability to transfer sperm. Males held under constant light were more severely affected than males held in darkness, but sperm transfer was not correlated with light intensity (between 80 and 3960 lux). Nearly all males held under constant light and most held under constant darkness were sterile or partially sterile. In either case, sterility was caused by poor sperm transfer. Females with a full complement of sperm were fertile regardless of holding condition of male pupae. The sterilizing effect of constant light or darkness was overridden by cyclic temperature of 28°C for 16 h and 22°C for 8 h. At a photoperiod of 16:8 h, pupae held in constant 28°C transferred less sperm than those held at 25°C. Pupae held at constant 30°C were sterile, but those held at 30°C for 16 h and 25°C for 8 h transferred similar quantities of sperm as males held at constant 28°C.