Vegetative filament apices of Vaucheria which differentiate into zoospores under altered culture conditions have been examined in sections prepared for light and electron microscopy. Soon after the migration and accumulation of cytoplasmic organelles into the filament tip, the centriole pair associated with each nucleus begins to form internal flagella. The flagella and their nuclei converge into many internal flagellar pools which then migrate to the surface of the zoospore and become part of the plasma membrane. These events in the filament apex take place during the septation of the vegetative filament. At the time of zoospore release, all nuclei are positioned just beneath the zoospore surface and are intimately associated with pairs of external flagella.