Throughout the winter, the northern house mosquito, Culex pipiens, is continually exposed to desiccating conditions. In this study, we report the water balance characteristics of diapausing and nondiapausing Cx. pipiens. To counter dehydration, female mosquitoes reared under conditions that induce diapause have lower water content, a higher dry mass and suppressed water loss in comparison to those reared in a nondiapausing environment. No differences were noted in dehydration tolerance, indicating only minimal changes in the overall amount of water available for exchange. Behaviorally, clustering of the mosquitoes reduced water loss in groups as small as five. This mosquito does not utilize water vapor to replenish internal water stores, but instead relies solely on drinking liquid water. Males showed no differences in their water balance characteristics when reared under diapausing or nondiapausing conditions, and this is consistent with their inability to enter diapause. The critical transition temperature, 35-40ºC, was the same for nondiapausing and diapausing mosquitoes. Overall and epicuticular lipid contents were nearly 3x higher in the diapausing mosquitoes, and we are currently analyzing qualitative changes in the lipid profiles. No differences in trehalose, sorbitol or total sugar content were noted during diapause, and likewise, preliminary studies indicate no increase of glycerol for diapausing mosquitoes. In conclusion, the reduced water loss of diapausing Cx. pipiens is likely a combination of the increased lipid content, particularly the cuticular lipids, and a reduction in the rates of respiration.