The relationship between body size and parity status of adult female Culiseta melanura collected from 3 locations in northeastern United States was studied by measuring wing lengths and examining ovaries of individual mosquitoes. Virus isolation was attempted from Cs. melanura collected in Maryland and in New Jersey. At all 3 locations, the size of Cs. melanura collected varied from large in the spring, to small in the summer. In New Jersey and Maryland mosquitoes collected in the fall were again large. The size of Massachusetts mosquitoes collected in the summer versus the fall was not different. In general, parous mosquitoes were larger than nulliparous mosquitoes in the spring but smaller than nulliparous ones in the fall. Eastern equine encephalomyelitis (EEE) and Highlands J (HJ) viruses were recovered from Cs. melanura only during the late summer when mosquitoes were small or during the fall months when larger mosquitoes were collected. We conclude that there is no detectable association between Cs. melanura size and parity status and that there is no obvious effect of mosquito size on EEE or HJ virus transmission.