Development of eupyrene sperm was inhibited for as long as 7 days after adult emergence in Ephestia (=Cadra) cautella (Walker) and Plodia interpunctell (Huebner) exposed as prepupae or as 1-day- old pupae to a sublethal temperature of 35 C for 3 days or more; 4- day-old or older pupae were unaffected. A temperature of 36 C was lethal to pupae of both species. Because eupyrene sperm is essential for oviposition and fertility, egg production in moths that were exposed to sublethal high temperatures was severely reduced. Mating behavior was unaffected.