SUMMARY: The nutritional requirements of thephagotrophic phytoflagellate Poterio-chromonas stipitata are defined. Minerals, glucose, ammonium-nitrogen and the vitamins thiamine, biotin and vitamin B12 are essential for the growth of this chrysomonad. The addition of L-isoleucine, L-leucine, L-valine and L-phenylalanine increases the yield of organism at least fivefold. Further augmentation with adenine, L-glutamic acid, L-histidine and L-arginine provides optimal nutrition for the organism.