Membranes are intimately involved in the life-cycle of nuclear polyhedrosis viruses (NPV), and the related (Bellett, 1969) granulosis viruses (GV). Generally, two different types of membrane are seen, which we categorize as envelopes and ‘transport’ membranes. The former represent those membranes which closely surround mature, occluded, nucleocapsids and which are also involved in the initial membrane fusion event thought necessary for virus uptake by midgut epithelial cells (Harrap, 1970; Summers, 1971; Kawanishi et al. 1972). The acquisition of envelopes is also a necessary requisite for occlusion (Summers & Arnott, 1969), a process that usually occurs only in tissues other than the gut. ‘Transport’ membranes, on the other hand, are involved in the movement of non-occluded nucleocapsids from the site of primary (gut) infection to other tissues, such as fat body, in which the progeny of a second round of virus replication become occluded; unlike envelopes, these membranes are often vesicular in profile (Harrap, 1970; Summers, 1971).