The shell-inhabiting, marine alga Ostreobium quekettii Bornet & Flahault is a distinct species reproducing by zoospores with four flagella. Sporangium formation and the development of zoospores are described. The material obtained from Helgoland (North Sea) is assumed to be identical with the type specimen from the French Atlantic coast.O. quekettii is said to be of worldwide distribution; in the absence of further information, it must be doubted whether other finds reported to belong to this species are identical with the type species.