Isometric, viruslike particles (VLP), 20–25 nm in diameter, were consistently found in some organs of the green peach aphids, Myzus persicae, from a stock colony of the Virus Department, Instituto Agronomico, Campinas, and of the Department of Plant Biology, University of Brasilia. They are presumed to be identical to those described by Peters (1967) in extracts of M. persicae in the Netherlands and considered to be a latent virus of this aphid. VLP were detected only in aphids up to 3 weeks old, even when newborn nymphs were immediately separated from the colony after their birth and raised separately. They were found initially in the midgut and, about 1 week later, in the nerve ganglia and salivary gland but not in other organs. In the midgut, VLP were noticed in the lumen, associated with the microvilli, or in the epithelial cells within membrane-bound cavities, basal infoldings of the plasmalemma, and embedded in the basal membrane. Clusters of vesicles associated with the presence of VLP were commonly seen in the cytoplasm. In the nervous ganglia, VLP often occurred forming crystalline aggregates of various sizes within the internal mass of axons and glial processes. Only rarely, particles were noticed in the peripheral neurons. In a few instances, VLP were also seen in the rhabdomere of the compound eye. In the salivary gland cells, VLP commonly appeared within deep infoldings of the plasmalemma at the basal part, forming a single layer of tight and hexagonally arranged particles. Less frequently, VLP were observed within secretion granules and large membrane-bound cavities. Histological observations suggest that these VLP might be transovarially transmitted. The midgut seems to be the main center of VLP multiplication, whereas the salivary gland and nervous ganglia appear to be preferential sites for their accumulation.