¹³¹I was added to seawater cultare medium and used to study the uptake of iodineby the brown algae (Laminaria japonica) fronds under laboratory conditions. Thefollowing results were rocorded: 1. Discs punched out of different zones of the frond of the algae were exposed to¹³¹I in the illumination and darkness for various lengths of tim. The experimentshowed that the ¹³¹I uptake value were reduced to the average by 33% in the darkness.It seemed that ¹³¹I uptake by seaweed is most likely the active process. 2. The efficiency of¹³¹I absorption of various parts ¹³¹I for 24 hrsobservation every two hours examined and recorded the ¹³¹I uptake. Two highor valuesoccured from 4: 30 to 6:30 a. m. and from 4: 30 to 6: 30 p. m. The preliminary resultshowed the rhythm of ¹³¹I uptake probably occured. 4. Lead, urea and copper would affect the ¹³¹I uptake which was accelerated by0. 5--2. 0 ppm of Pb, 0 .5--10 ppm of N, and 0. 01--0 .05 ppm of Cu respectivoly,but it was inhibited when the concentration is more than 0. 5 ppm of Cu or 20 ppm ofN. 5. It seemed to be a better method that the frond was cut into small discs in regu-lar order to determine the distribution of ¹³¹I in the fronds. 6. The distributlion of ¹³¹I in the fronds of the algae was observed. The absorbed was transported to the basal part during the first weed. The greatest values of theradioactive iodine in basal part appeared after the 3rd day. 7. The analysis of 99 discs from centre to margin of the fronds revealed that thecontent of ¹³¹I was higher in margin than in the central part. 8. Experimont showod that the redioactivity of the cortex of the stipe and base ofthe algae were richcr than that of the pith. 9. The loss curve of radioaetivity from discs of the seaweed is divided into twophase: an initial rapid lost, roughly 50% of abscrbed.¹³¹I was lost within 2 days; thenthe ¹³¹I lost slower. 10. L. japonica may be used as bioindicators of the radioactive poliution.