Introduction. The Baculoviridae is a large family of occluded viruses pathogenic for arthropods belonging predominantly to the insect orders Lepidoptera, Diptera and Hymenoptera. They have received considerable attention recently because of their potential for use as insecticides and for their potential use as vectors for the introduction and expression of foreign genes in insects and insect cell lines. Baculoviruses are characterized by a large rod-shaped, enveloped nucleocapsid, and a double-stranded supercoiled DNA genome of 88 to 160 kilobase pairs (kbp). Two subgroups of baculoviruses occlude their virions in large protein crystals (Fig. 1) which serve to stabilize the virus for many years in the environment. Subgroup A, the nuclear polyhedrosis viruses (NPVs), have many virions occluded within single intranuclear crystals, while Subgroup B, the granulosis viruses (GVs), normally have only a single virion within cytoplasmic crystals. Subgroup C baculoviruses are non-occluded.