The purpose of this investigation was to identify the mosquito (Diptera: Culicidae) vectors of West Nile virus (WNV; family Flaviviridae, genus Flavivirus) during an epizootic WNV outbreak in eastern Puerto Rico in 2007. In June 2006, 12 sentinel chicken pens with five chickens per pen were deployed in six types of habitats: herbaceous wetlands, mangrove forests, deciduous forests, evergreen forests, rural areas, and urban areas. Once WNV seroconversion in chickens was detected in June 2007, we began trapping mosquitoes using Centers for Disease Control and Prevention (CDC) miniature (light/CO₂-baited) traps, CMT-20 collapsible mosquito (CO₂- and ISCA SkinLure-baited) traps, and CDC gravid (hay infusion-baited) traps. We placed the CDC miniature traps both 2–4 m and >30 m from the chicken pens, the collapsible traps 2–4 m from the pens, and the gravid traps in backyards of houses with sentinel chicken pens and in a wetland adjacent to an urban area. We found numerous blood-engorged mosquitoes in the traps nearest to the sentinel chickens and reasoned that any such mosquitoes with a disseminated WNV infection likely served as vectors for the transmission of WNV to the sentinels. We used reverse transcriptase-polymerase chain reaction and isolation (C636) on pools of heads, thoraxes/abdomens, and legs of collected blood-engorged mosquitoes to determine whether the mosquitoes carried WNV. We detected WNV-disseminated infections in and obtained WNV isolates from Culex nigripalpus Theo (minimum infection rate [MIR] 1.1–9.7/1,000), Culex bahamensis Dyar and Knab (MIR 1.8–6.0/1,000), and Aedes taeniorhynchus (Wied.) (MIR 0.34–0.36/1,000). WNV was also identified in and isolated from the pool of thoraxes and abdomens of Culex quinquefasciatus Say (4.17/1,000) and identified in one pool of thoraxes and abdomens of Culex habilitator Dyar and Knab (13.39/1,000). Accumulated evidence since 2002 suggests that WNV has not become endemic in Puerto Rico.