Quadriflagellate zoospores and conditions for their induction are described for an algal isolate tentatively identified as Cylindrocapsa geminella Wolle. Previous to this report, only biflagellate zoospores were known for Cylindrocapsa while quadriflagellate zoospores were thought to characterize the closely related Cylindrocapsopsis; this distinction is no longer valid. In our isolate, a vegetative cell may differentiate directly into a single zoospore or, more commonly, zoosporogenesis is preceded by division of a vegetative cell into two, four, or eight daughter cells, each of which becomes a zoospore. Variation in zoospore arrangement depends on the number and nature of the division sequences. Ultimately, zoospores are released from the more-or-less dissociated parental cell wall in one or more vesicles. Each primary vesicle contains one, two, four, or occasionally eight zoospores; zoospore release follows the gradual distention and dissolution of the enclosing vesicle. Light microscopic observations suggest that the zoospore-containing vesicles arise from altered cell wall material. Zoospore germlings and variations in the appearance of vegetative filaments are aiso described and attention is called to the nature of the cell wall, which is quite unlike that of most other filamentous green algae.