Subculture from unialgal culture of Nitaschia closterium,kept in our laboratory for manyyears, was made first in enriched sea water under suitable conditions for a week to gethealthy diatoms for inoculum. These diatoms were then carefully washed by centrifugingand subcultured in sterilized natural sea water,not enriched and very deficient in phosphate,with8 ×10⁶ to 10×10⁶ cells per ml., for ten hours before inoculating the cultures for experi-ments. Dead cells occurred if this starving culture lasted too long or contained too many cells.Experiments were all carried out in 500 ml. conical flasks each containing 200 ml. sterilizednatural sea water enriched with NO_3-N, 4 p.p.m. and P³² 20μc. Cultures of cach serieswere illuminated by a 1,000 W.bulb.Diatoms were washed 5 times by centrifuging to get ridof the P³² adsorbed on the diatom frustules, before P³² absorbed being measured by a Geigercounter. 1. Influences of different temperatures on the absorption of P³². Cultures kept at 0, 5, 10, 15, 20, 25 and 30℃, were arranged obliquely under a 1,000W.bulb at a distance of 53 cm.measured from the surface of the bulb to the center of the bot-tom of the culture flask situated just inside a wall of white glazed paper which was usedto reflect the light. All cultures lasted 12 hours. Diatom cells in cultures at 15℃ absorb largest amount of P³³ (Fig. 1,A), the increase ofdiatom numbers here is also the largest (Fig. 1, B), being up to 44.1%, and the amountof P³² absorbed by a single diatom cell is the largest as well. The next come in order arecultures at 20℃ and 10℃, the smallest absorption occurring in cultures at 5℃ and 0℃. 2. Influence of different illuminating periods on the absorption of P³². Cultures were kept at 15℃ and arranged straight beneath the 1,000 W.bulb with a dis-tance of 33. 5 cm. with light reflecting arrangement; all lasted 26 hours. Three series ofcultures, inoculated respectively with 6×10⁶, 8×10⁶ and 9×10⁶ cells per ml., were made,each consisting of 4 groups, A, B, C and D: A. 26 hours' illumination. The absorption of P³² in 5 minutes gives a count of 100 per minute, increased to morethan 2,000 per minute in 8-9 hours, and then the absorption rate increases rapidly up to 12,800 counts per millute in 17 hours for cultures inoculated with 6×10⁶ cells per ml. (Fig.2a,A), and up to 12,000 and 8,600 counts per minute in 20 and 21.5 hours for culturesinoculated with 8×10⁶ and 9×10⁶ cells per ml.respectively (Figs.2b, A 2c, A).The summitof the absorption curve is higher and reached earlier in cultures with a smaller inoculum. B. 14 hours' illumination followed by 12 hours' darkness. During the period of illumination the absorption curve (Fig. 2, B) follows more or lessthe course of the curve for 26 hours' illumination (Fig. 2,A). When the black-out starts,the absorption curve of cultures inoculated with 6×10⁶ cells per ml. begins to drop im-mediately (Fig. 2a, B), while the curves of cultures inoculated with 8×10⁶ and 9×10⁶cells per ml.still going upward for another three and six hours respectively before dropping(Fig. 2 b, B 2 c, B). C. 12 hours' illumination after 14 hours' darkness. The absorption is very weak during the 14 hours' black-out period, giving only 100-200counts per minute and gradually increases after illumination being started, reaching gteatspeed after 6 hours' illumination (Fig. 2, C). D. 26 hours' black-out. Absorption is very weak throughout the 26 hours, being only 100-200 counts perminute. The following phenomena are worth noticing in the above illumination experiments: 1. Greatest absorption speed of P³² generally occure 8-12 (14) hours after illuminationbeing started. 2. The summit of the absorption curve is reached earlier in the culture with smaller ino-culum, possibly as a result of less sheltering effect and hence more light being received byeach cell. 3. The absorption curve drops after the summit is resched, as a result of P³² being se-creted out into the culture medium.