The effect of light intensity, temperature, pH and density of sea water in thephotosynthetic activity of carpores and free-living filaments was studied by usingthe oxygen electrode method. The results obtained are summarized as follows:1. The photosynthetic activity of carpospores depends on the time of exposure tolight; the activity of carpospores those not exposed to light is much lower than thoseexposed to light for 1--3 days.2. The activity of free--living filaments shows periodic alternation when culturedunder 12hr/12hr light-dark cycle, with the peak value in the middle of the lightphase and the valley in the middle of the dark phase.3. The activity is greatly raised when 5mM NaHCO_3 is added to sea water, indi-cating the need for standardizing the measuring condition.4. The activity of both carppores and the filaments shows a linear relationship- to light intensity over the range from 500 to 7000 Lux. While the activity of the fila-ments decreases when the intensity above 8000 Lux, whereas that of carpospores doesnot fall until 12,000 Lux.5. Within the temperature range of 10--25℃, the activity of carpospores andfilaments increases.6. The activity of carpospores and the filaments reaches to the hightest point atpH 8.0, less at 7.5 and 8.5, at 7.0 and 9.0 reaches to the lowest.7. For both stages, the highest activity appears at the sea water density of1.025.The resuls obtained are consistent with the germinating rate of carpospores andthe growth rate of the filaments. as well as the ecological observation.