A study of thin sections of nuclei infected with a polyhedral virus in an insect suggests a cycle of virus development commencing with the attachment of rod-shaped particles to strands of chromatin. Apparently, the chromatin is then converted to virus, in form of minute spherical bodies surrounded by membranes, that increase in size to form rods. The rods may escape from their developmental membranes to repeat the cycle, or rods and spheres may be occluded by protein material to form polyhedra in which case virus development ceases.