West Nile virus (WNW) was initially detected in North Dakota in 2002. In 2003, North Dakota had one of the highest prevalence rates of human disease, but disease was significantly reduced in 2004. Mosquitoes were trapped in 2003 and 2004, frozen on dry ice, and shipped to Connecticut where all mosquitoes were identified to species and tested for virus in Vero cells. Isolates were identified by TaqMan RT-PCR. Twenty-two and 35 species of mosquitoes were identified and tested in 2003 and 2004, respectively. WNV was isolated from 1, 15, and 28 pools of Ochlerotatus triseriatus, Culex tarsalis, and Aedes vexans, respectively in 2003, and from 2, 7, and 29 pools of Ochlerotatus trivitattus, Aedes vexans, and Culex tarsalis in 2004. Isolations were made from mosquitoes captured from the first week of July through the last week of August. Culex tarsalis and Aedes vexans appear to be the most important vectors.