Upper temperature limits were established for beet armyworm moths,Spodoptera exigua(Hübner) (Noctuidae), under controlled humidity–air flow conditions. Preliminary tests determined that a one-hour exposure to 45°C resulted in an approximately 50% reduction in the number of moths capable of flight following a 24-h recovery period. Subsequent tests using 45°C as the stress temperature indicated a rapid decline in thermal resistance of moths with increasing age. Moths maintained at each of 3 temperature regimes (22, 27, or 32°C) for 72 h post-emergence were inconsistent in their flight capabilities following exposure to the stress temperature. Developmentally acclimated moths whose pre-adult stages were reared at these 3 temperatures showed decreased heat resistance at the lower temperature, but inconsistent responses at the higher temperature. Flight capability of irradiated moths in the absence of heat stress was unaffected; however, flight capability following exposure to 45°C significantly decreased as dosage (20, 40, 80 krads) increased