Larvae, early and late (pharate) pupae, and adults ofDiatraea saccharalis(F.), were subjected to various constant high temperatures ranging from 35 to 43°C and fluctuating temperatures from 35–21 to 43–21°C, to evaluate effects upon survival and further development. Constant temperature was consistently more detrimental than fluctuating temperature, regardless of life stage. Late pupae were the least susceptible, and comparatively susceptible larvae and adults were considered to be the limiting stage in nature. Laboratory findings were compared with short-term daily in-the-field high temperatures of up to 46°C in Puerto Rico, and their significance to control efforts is discussed.