The growth of pure cultures ofDutialiella euchlorastrains WHOI-1 and WHOI-2,Platymonas subcordiformis,Phaeodactylum tricormitum,Chlorella vulgaris, andCyclotella meneghinianawas compared in two enriched sea water media and two artificial marine media. Three general patterns of growth were observed; the green flagellates grew the same in all media,PhaeodactylumandChlorellagrew more slowly in the artificial media, andCyclotelladid not grow in the artificial medium without organic supplements. The addition of vitamin B12to the artificial medium promoted good growth ofCyclotella, and of the species studied this alga was the only auxotroph. Growth in the enriched sea water medium was not affected by a precipitate, but removal of the precipitate markedly affected the rate of growth and the total number of cells obtainable.