The influence of low temperature on the survival and geographical distribution ofDiatraea grandiosellawas examined. Data from both climatological and laboratory observations indicated that low-temperature mortality of diapause larvae is the principal reason that the species has not colonized northern corn-growing states within the U.S.A. Results showed that diapause larvae are susceptible to freezing and cannot successfully overwinter in field conditions where the 10-year-mean January temperature falls below −7°C. This finding correlated with information obtained about the species' coldhardiness. Diapause larvae were found to freeze at ca. −2°C and to supercool to only ca. −8.5°C. Severe mortality occurred when larvae were exposed to −4°C for 2 or 4 weeks. These results provide a basis for predicting the eventual distribution of the species within the United States.