(1) Effect of variation of day-length, day- and night-temperatures and of intensity of daylight upon the growth rate of Chlorella was investigated using combinations of conditions which were varied as follows:Day-length: 6hours (18 hours dark), 12 hours (12 hours dark), 18 hours (6 hours dark), and 24 hours (no darkness).Day-and night-temperatures: 25°, 15° and 7°C.Intensity of daylight: 50, 10, 2, and 0.4 kilolux.(2) In general the rate of growth was affected by changes of day-temperature by far more profoundly than by changes of night-temperature. In the temperature range studied, the higher the day-temperature, the greater was the growth rate. Higher night-temperatures had no recognizably favorable effect except when the day-temperature was as low as 7°. (3) In general, the growth rate was directly proportional to the day-length (day-limited growth) at shorter day-lengths. Such a proportionality extended to longer day-lengths, the lower the daylight intensity; and the intensity of daylight, under which a day-limited growth occurred markedly, was higher, the higher the day-temperature. At longer day-lengths, and especially under stronger daylight, the growth rate tended to become independent of day-length (the phenomenon of day-saturation), or to become rather smaller with the increase of day-length (the phenomenon of day-oversaturation). The latter phenomenon was accompanied by a bleaching of algal cells. Both the day-saturation and -oversaturation occurred ceteris paribus more markedly at lower day-temperatures. The day-oversaturation occurred most profoundly when both the day and night temperatures were 7°, but ceased to occur when the day-temperature was 7° and the night-temperature was 25°.(4) Although the growth rate increased, in general, with the increase of day-length, the favorable effect of long-day condition decreased with the decrease of day-temperature. At 7°, and especially when the daylight intensity was high, there was almost no difference in the growth rates under short-day and long-day conditions. It was also observed that the temperature-dependence of growth rate decreased and eventually tended to disappear with the decrease of daylight intensity and with the shortening of day-length, and that the daylight intensity, under which the growth rate became light-saturated, was lower at lower day-temperatures. All these facts indicate that the short-day condition and weaker daylight become relatively less disadvantageous with the decrease of day-temperature. (5) The phenomena of day-limited and day-saturated growths as they were conditioned by temperature, intensity of daylight and day-length were explained on the basis of observations made earlier by Tamiya et al., who, by determining the relative rates of light-dependent and light-independent phases in the growth process of algae, have found that the light-independent phase has a considerably greater temperature-coefficient than the light-dependent phase.(6) The phenomenon of day-oversaturation, which occurred at lower temperatures and under stronger daylight, was explained as being due to the injurious effect of strong upon the so-called light cells which have been shown by Tamiya et al, to become abundant in cultures when temperature was low and was strong.