AbstractThe effect of a range of externally supplied carbon Compounds on the growth ofPhaeodactylum tricornutumBohlin in the dark and in the light has been studied. No substance tested was capable of supporting growth of the diatom in The dark and it appears that the organism is obligately phototropic. P. tricornutum is able to respire glucose, mannitol and lactate without the energy released being apparently available for growth. Some slight stimulation of growth occurs in The light and this appears to be partly due to the occurrence of trace element impurities in the organic compounds.