The developmental cytology of the vegetative filament of Vaucheria has been investigated with light and electron microscopes. The vegetative filament consists of three distinct zones: the apical zone, the sub-apical zone and the zone of vacuolation. Organelle distribution and associations in these zones have a primary role in controlling morphogenetic events which influence growth and induce differentiation of sexual and asexual organs. Cell elongation in the polarised vegetative filament occurs by vesicular addition in the apical zone. Two types of active cyclosis are found exclusively in the zone of vacuolation. Nuclear cyclosis involves microtubular bands which associate with persistent centrioles found adjacent to the outer nuclear membrane. A specialised mitochondrial-dictyosome association is transported by a microfilament-like system of anastomosing strands. This latter system is extremely labile to conventional fixatives.