We tested the hypothesis that mild winter temperatures are detrimental to the survival and reproductive potential of insects. We measured survival, body size, and potential fecundity of a freeze tolerant insect, the goldenrod gall fly (Eurosta solidaginis), after overwintering in the laboratory for ~3 mo. frozen at -22 degrees C, unfrozen at 0 degrees C, or unfrozen at 12 degrees C. Larvae held at 12 degrees C suffered high mortality (70%) and relatively low potential fecundity as adults (mean+/-SEM=199+/-11 eggs/female), while those held at 0 degrees C had both low mortality (11%) and high potential fecundity (256+/-15 eggs/female). Freezing (-22 degrees C) increased mortality (30% overall) but did not significantly reduce fecundity (245+/-13 eggs/female). Egg length and width were constant regardless of treatment group or female body size. Analysis of covariance indicated that reduced fecundity in the 12 degrees C group was related to reduced larval body weight following treatment. Patterns of larval weight loss in the experimental treatments were generally correlated with previous reports of latitudinal trends in weight loss through the winter. We conclude that mild winter temperatures may be detrimental to some overwintering insects, particularly species that do not feed following winter diapause. Low temperature and even freezing are beneficial, allowing conservation of energy reserves to maintain high survival and potential fecundity.