The labyrinthula isolates of Watson will grow over a wide range of pH values in a medium of which the inorganic constituents reflect their marine origin. All are obligately marine, though the less exacting requirements of L. macro-cystis var. atlantica suggest an estuarine habitat. Although only L. minttta var. atlantica requires an amino acid (l-leucine) as a growth factor, no isolate grew well on substrates other than amino acids. All require thiamine. Organic requirements may be obtained from host plants.