Growth rates of Monochrysis lutheri Droop increased with light intensity in seawater medium (SWM), whereas cells grown at either low NaCl (0.013 M) or high NaCl (1.02 M) showed reduced growth at higher light intensities with an optimal intensity near 0 05 g cal/cm2 per min.Chlorophyll and total carotenoids usually declined with increasing light intensity except in cells maintained on a high salt regime, when the carotenoid content remained high at all light intensities.The concentration of intracellular cyclohexanetetrol in SWM cells was approximately 0.3 M and was dependent upon the osmotic properties of the medium. Added NaCl, mannitol, sucrose, but not glycerol, caused accumulation of the cyclitol, whereas dilution of the medium caused its expulsion from the cells. Addition of 0.5 moles/liter of NaCl to SWM cells caused an 80–90% increase in cyclitol concentration within 4 hr; dilution of the medium reduced the cyclitol to the new steady state level within 10 min.