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Comparisons were made between groups of Culex pipiens L. with different physiologic histories to test their ability to sucessfully overwinter under field conditions. On 14 December 1978, each group of mosquitoes was marked with a distinctive fluorescent dust and released inside an abandoned ammunition bunker at Fort Washington, Maryland. To insure that dead mosquitoes could be dissected and information obtained on their ovarian development, a sample of females from each group was also released into a plexiglass cage that was attached to the inside wall of the room. The physiologic histories of each group of mosquitoes were as follows: (a) “wild caught,” those which had entered the bunker prior to the release date, (b) “lab-reared diapausing nonblood-fed,” (c) “lab-reared diapausing blood-fed nongravid,” (d) “lab-reared diapausing blood-fed gravid,” (e) “lab-reared nondiapausing nonblood-fed,” and (f) “lab-reared nondiapausing blood-fed.” By 8 March 1979, all of the lab-reared nondiapausing groups, of mosquitoes released in the room had died, whereas 15.7, 22.4 and 24.7% were recovered from the “lab-reared diapausing nonblood-fed,” “lab-reared diapausing blood-fed” (gravid and nongravid) and “wild caught” mosquitoes, respectively. For the mosquitoes in the cage, only 0, 2.1 and 7.0% of the “lab-reared nondiapausing blood-fed,” “lab-reared nondiapausing nonblood-fed” and “lab-reared diapausing blood-fed gravid,” respectively, survived. This compared to 45.4, 56.8 and 58.0%, respectively, for the “lab-reared diapausing nonblood-fed,” “lab-reared diapausing blood-fed nongravid” and the “wild caught” groups. These data provide evidence to support the theory that a significant number of diapausing Cx. pipiens which have taken a prehibernation (possibly viremic) blood meal do not develop eggs and can survive the winter at rates comparable to diapausing nonblood-fed mosquitoes.