The growth cycle of free-living Gymnodinium microadriaticum consisted of the following stages. Vegetative cells on the periphery of a group of cells gave rise to a zoosporangium. The zoospore broke out of the surrounding membrane as a motile gymnodinioid cell possessing longitudinal and transverse flagella and released a refractile body. After a period of free swimming the zoospore came to rest, shed its flagella and rounded up to become a vegetative cell again, which eventually underwent binary fission within the parent cell membrane to form two daughter cells. These daughter cells eventually became gymnodinioid zoospores again. Vegetative cells within a group of cells formed two non-motile autospores by binary fission.