Larvae, early and late (pharate) pupae, and adults ofDiatraea saccharalis(F.) were subjected to various high temperatures ranging from 35 to 43°C and fluctuating temperatures from 35–21 to 43–21°C. Adults derived from the immatures and the heat–treated adults were outcrossed to normal partners to determine effects on mating success, fecundity, and fertility. Mating success of heat-treated adults was severely affected as length of treatment and temperature increased. Mating success of adults derived from treated larvae and pupae was only slightly affected. Fecundity was unaffected among females derived from treated larvae and pupae. Fecundity was lower in heat-treated adult females only after 96-h exposure, but age was a major factor. Fertility of males derived from heat-treated larvae and pupae, as well as treated adult males, was not affected. Females from larvae exposed to 39–21 and 36.5°C and higher temperature suffered substantial fertility loss. Among females derived from heat-treated pupae, all temperature levels caused some fertility loss, and early pupae females showed a higher degree of fertility loss than late pupae. Hatch of eggs, once fertilized, was not reduced.