The infectivity and pathogenicity of newly discovered baculovirus, CuniNPV (family Baculoviridae, genus Nulclepolyhedrovirus) originally isolated from the mosquito Culex nigripalpus Theobald, was evaluated in laboratory bioassys against thirteen species and four genera of mosquitoes native to the northeastern U.S. Purified virus at a dosage rate of 1.6 × 10⁷ occlusion bodies/ml with 10 mM Mg² added was used in exposures with second through fourth instars at temperatures ranging from 17 to 27°C. High infection rates and accompanying mortality were achieved in Cx. pipiens L. (83.0–14.4%), Cx. pipiens f. molestus (80.4% infection), and Cx. salinarius Coquillett (48.0–43.1%). Cx. restuans Theobald was also susceptible but infection rates were lower (21.3–12.5%). The gross pathology associated with infection was identical to that reported in Cx. nigripalpus. Infected larvae were lethargic and were often suspended at the water surface. Development of CuiNPV was observed in the nuclei of the midgut epitheial cells in the gastric caeca and posterior region of the stomach of host larvae. One hundred percent mortality was observed in all larvae that exhibited gross symptoms of infection within 4-d p.i. Cx. territans Walker (subgenus Neoculex Dyar) was the only Culex mosquito that was not susceptible. No infections were obtained with any species of Aedes [Ae. vexans (Meigen)], Culiseta [Culiseta morsitans (Theobald)] or Ochlerotatus [Ochlerotatus canadensis (Theobald), Oc. cantator (Coquillett), Oc. communis (De Geer), Oc. excrucians (Walker), Oc. japonicus (Theobald), Ochlerotatus stimulans (Walker), and Ochlerotatus triseriatus (Coquillett)]. The host range of CuniNPV appears to be restricted to Culex mosquitoes within the subgenus Culex. An inhibitory effect on transmission of CuniNPV was observed when a liver powder/Brewer’s yeast mixture was used as a source of food reinforcing the critical role of Mg² and sensitivity of the infection process to the presence other divalent cations (Cu² , Fe² , and Zn² ) in the larval medium that interfered with the infection process. The high infectivity and pathogenicity of CuniNPV for the principal vectors of West Nile virus in North America make CuniNPV an attractive candidate for future development as a biopesticide.