In the course of experimental infection of Penaeus monodon with yellow-head virus (YHV) for virus isolation and purification, 1 batch of prawns yielded hemolymph fractions dominated by a previously undescribed non-occluded baculovirus rather than YHV. Injection of test shrimp with a semi-purified preparation of this virus gave rapid mortality, and examination with the transmission electron microscope revealed a dual infection where cells containing the new virus dominated, but some cells containing YHV could also be seen. The tissues infected by the 2 viruses were similar. However, in contrast to YHV, the new virus was assembled completely in the nucleus and in the absence of occluding protein (polyhedrin). By normal histology, the most characteristic feature of infection was eosinophilic Cowdry A-type inclusions in hypertrophied nuclei with marginated chromatin, especially in epithelial cells of the stomach. These intranuclear inclusions became lightly basophilic in late stages of infection. In the epithelial cells of the gills, ultrastructural pathology included nuclear hypertrophy and cytoplasmic disintegration leading to large voids at lysed cell sites. By negative staining, completely assembled, enveloped virions were ellipsoid to obovate with a distinctive multifibrillar appendage and they measured 276 x 121 nm (excluding the appendage). Enveloped and unenveloped nucleocapsids were significantly different in size, indicating possible shortening and thickening of the viral core and nucleocapsid during viral assembly. Isolation and purification of the nucleic acid from the new virus yielded double-stranded DNA of approximately 168 kilo base pairs. This DNA did not cross-hybridize with DNA fragments isolated from YHV-infected shrimp or from monodon baculovirus (MBV). The features placed this virus in the family Baculoviridae, subfamily Nudibaculovirinae as PmNOBII, but for convenience we have named it informally as Systemic Ectodermal and Mesodermal Baculovirus (SEMBV).