SUMMARY: Labyrinthula vitellina var. pacifica requires a steroid as growth factor. Cholesterol, δ4-cholestenone, fucosterol, and β-sitosterol are active. When cholesterol is modified by (1) the C3‡ hydroxyl group becoming trans to the C10 methyl group, (2) esterification of the C3 hydroxyl group, or (3) saturation of the C5, 6 double bond, the resulting compound is inactive. Modification of the side chain of cholesterol or cholestenone by the addition of a C24 ethyl or vinyl group does not destroy activity but unsaturation at C22, 23 does, as do the more marked changes found in diosgenin, deoxycorticosterone acetate, and methyltestosterone. Bile salts are inactive. The specificity of the requirement indicates that active steroids function as essential metabolites rather than solely as protective agents. Active steroids may be obtained by this micro-organism in nature from host plants or from diatoms and bacteria associated with the hosts.