Potosi virus (POTV) (Bunyaviridae: Orthobunyavirus) was first isolated from Aedes albopictus (Skuse) collected in Potosi, MO, in 1989, and subsequent isolations were reported from Illinois, Michigan, Ohio, and the Carolinas. To determine whether the distribution of this virus extends into the northeastern United States, we analyzed arboviruses acquired from mosquitoes collected in Connecticut from 1998 to 2004. In 2001, a bunyavirus was isolated from Aedes vexans (Meigen) that was different from other arboviruses known to occur in Connecticut by cross-neutralization and reverse transcription-polymerase chain reaction (RT-PCR) assays. Nucleotide and encoded amino acid sequences of a portion of the G2 envelope gene were 99 and 100% similar to POTV, respectively, yet distinct from indigenous strains of Jamestown Canyon (JCV), Cache Valley (CVV), and Trivittatus virus (TVTV). Viral isolates obtained from the statewide surveillance program were retested by RT-PCR coupled with restriction enzyme analysis to distinguish POTV from other bunyaviruses. POTV isolates, previously typed by neutralization, were correctly identified by RT-PCR; however, many isolates classified as JCV or CVV by enzyme-linked immunosorbent assay proved to be POTV by molecular assays. In total, 92 strains of POTV were isolated from 12 mosquito species in 2000, 2001, and 2003, whereas POTV was not detected in mosquitoes sampled during 1998, 1999, 2002, and 2004. Viral isolation rates were highest for Anopheles punctipennis (Say) (3.2–11.3 infection rate per 1,000 mosquitoes), whereas the greatest number of isolates came from Ochlerotatus trivittatus (Coquillett) (8–16 isolates). This finding represents the first detection of POTV in the northeastern United States where it infects a diverse array of mosquito species.