In dengue virus infected Aedes albopictus cells, electron-dense particles, larger than single ribosomes, were arranged on the cytoplasmic sides of rough endoplasmic reticulum (RER) membranes. Mature virions 40--45 nm in diameter as well as vesiculotubular structures 50--120 nm in diameter appeared in enlarged cisternae of RER filled with fine granular substance. Many of the mature virions and somewhat degenerated vesiculotubular structures remained to be enclosed in membranous structures presumably derived from RER, even after degeneration of infected cells. The findings suggest that development of dengue viruses in cultured A. albopictus cells takes place in close relationship with the activated membranes of RER. Other morphological changes observed in dengue infected A. albopictus cells were 1. electron-dense "double-track structures" in areas of virion morphogenesis, 2. fine crystalline structures in type-2 dengue infected cells, and 3. aggregates of nucleoid structures, in cells persistently infected with type 2 dengue virus. The implication and nature of these structures in relation to virion morphogenesis remain to be investigated.