To find out if insects will survive the low temperatures, reduce pressure, and low humidity in the stratosphere where jet aircraft operate, several species were exposed in the unpressurized and essentially unheated sections of jet fighters and bombers during flights. A high mortality was obtained in 45- to 55-minute high-level flights of jet fighters except in the radar compartment, where heat from electronic equipment gave protection. In a jet bomber all the insects were killed on a 3-hour flight at 40,000 feet (outside air temperature -51°C.) and in two of three flights of 2 to 2 hours at 35,000 to 40,000 feet. Shorter flights at that altitude or long flights at low altitudes gave reduced kills.