The overwintering ecology of Culiseta melanura was studied in a seasonally flooded evergreen forest swamp in south central Connecticut in an effort to clarify which larval stages successfully overwinter in the northeastern USA, and to determine the degree to which larval development and/or mortality occur during the winter months. A total of 8,626 immature Cs. melanura were collected weekly for analysis from subterranean crypts and cavities located under the roots of trees from December 13, 2011 to May 31, 2012. Despite the formation of ice on the surface water at the entrance holes to the crypts, water temperatures within the cavities remained above freezing (average  =  1.8°C) throughout the coldest winter months of January and February. A heterogeneous population of 2nd, 3rd, and 4th instars were recovered throughout the winter and early spring in the same relative proportions (30%, 30%, 40%, respectively), with no significant change in their comparative abundance during this period, providing unequivocal evidence that all 3 instars successfully overwinter in the region. Findings further demonstrate that larvae undergo no development during the winter and do not appear to be impacted by any measurable mortality. The cessation of larval diapause and a resumption of development was observed in mid-April and was coincident with a gradual increase in water temperature within the crypts to 9°C, in agreement with a previously calculated developmental thermal minimum of 8.5°C for Cs. melanura. This resulted in a protracted period of pupation that encompassed a minimum of 5 wk, followed by a staggered emergence of adults and an overlap of the residual overwintering population with larvae of the 1st summer generation.