The black cutworm, Agrotis ipsilon (Hufnagel) (Lepidoptera: Noctuidae), is a serious localized pest of vegetable and field crops. We have characterized a newly discovered baculovirus, the Agrotis ipsilonmulticapsid nucleopolyhedrovirus (AgipMNPV), that was isolated from A. ipsilon in Illinois. Restriction enzyme fragment profiles of AgipMNPV DNA were distinct from those of previously described nucleopolyhedroviruses. Electron microscopy of AgipMNPV-infected tissues indicated that nucleocapsids of this virus are multiply enveloped. A. ipsilon was highly susceptible to infection by AgipMNPV and significantly more susceptible to infection by AgipMNPV than by Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV). Host range studies showed that Heliothis virescens and Helicoverpa zeawere moderately susceptible to infection; Pseudaletia unipuncta and Spodoptera frugiperda were only partially susceptible, and Anticarsia gemmatalis, Spodoptera exigua, Trichoplusia ni, and Ostrinia nubilaliswere not susceptible to infection by AgipMNPV. Because of its high virulence, AgipMNPV has potential as an alternative to chemical insecticides for control of A. ipsilon.