Before cold acclimation, diapausing mature larvae ofHomoeosoma electellum(Hulst) have glycogen levels two times greater than nondiapausing mature larvae, trehalose levels are 30 times greater, and larvae have a lower supercooling point by approximately 8°C. Significant increases in trehalose occurred in diapausing larvae kept at 5°C for 7 and 18 d and at 0°C after 10 d. In most cases, there was a sufficient concomitant reduction in glycogen to account for the trehalose increases. Nondiapausing larvae in the temperature treatment groups of 10, 5, and 0°Cshowed significantly greater trehalose levels over initial values; yet these levels are almost two times lower than initial levels in diapausing larvae and three times lower than the greatest levels achieved in this study by diapausing larvae cold-acclimated to 0°C. No specific relationship was found between temperature acclimation and supercooling capacity in diapausing or nondiapausing larvae. Nondiapausing larvae died after being held at −10°C for 1 d, whereas 44% of the diapausing larvae survived to pupation after 3 d of exposure.