Abstract Polyclonal antiserum produced against preoccluded virions from the Orgyia pseudotsugatamultinucleocapsid nuclear polyhedrosis virus (OpMNPV) was used to screen an OpMNPV λgt11 expression library. The insert from one of the immunoreactive phage isolates hybridized to OpMNPV orf86( p91), a 2460-bp (819 amino acids) open reading frame that encodes a predicted protein of 91 kDa. Antibodies generated against a maltose binding protein–P91 fusion detected a band of approximately 91 kDa on Western blots of extracts of OpMNPV-infected Lymantria disparcells. This band was first observed at 18 hr p.i. and was present at all later time points. Similar results using this antiserum were seen with a time course of Autographa californica-infected Spodoptera frugiperdacells. Localization of P91 by confocal immunofluorescence microscopy showed that the protein was concentrated near the nuclear membrane and at late times p.i. was most concentrated near polyhedra. Immunoelectron microscopy indicated that P91 was present in both the capsid and envelope surrounding the capsid of occlusion-derived virions. Fractionation studies employing NP-40 and Western blot analysis indicated that P91 was associated with the capsid structure.