The development of dengue viruses type 1 obtained from acute human sera and inoculated into mosquito cell cultures, was observed by standard transmission electron microscopy and cytochemical staining. It follows the trans-type mechanism already established of other dengue types. Direct passage of single virus particles across the cell membrane seems to be a pathway of entry and exit in dengue-1 infected cells. The nature of numerous electron translucent vesicles and tubules, produced simultaneously during virus replication inside the rough endoplasmic reticulum, was analyzed by cytochemical tests. The largest amount of virus particles was produced inside cell syncytia.