-~12- SECTIONII FURTHER CONTRIBUTIONS ON GROSS BETA RADIOACTIVITY OF PLANKTON AND BOTTOM SAMPLES AT RONGELAP ATOLL 1954-1958 INTRODUCTION Plankton in Rongelap lagoon took up large amounts of radioactivity from fallout following the Bravo detonation at Bikini Atoll on March 1, 1954. Since then,the radioactivity has declined with only relatively slight additions from the two succeeding series of tests, Redwing in 1956 and Hardtack in 1958. Since the last summary report on the Rongelap surveys (UWFL-43) was written, plankton has been collected four times, July 1956 and 1957, and March and August, 1958, and lagoon bottom samples once, in August 1958. The present report gives available data on plankton through 1958 including a reevaluation of 1954-1955 counts which were reported in UWFL-42:43; however, ash weight as well as wet weight is used, with a modification of results, and a comparison is made of activity yielded by fine-meshed as contrasted with coarse-meshed plankton nets. The rate of physical decay of early samples is compared with the rate of change, herein termed decline, of successive samplings at later dates, up to more than four years after the original fallout.