-~14the pre-Mike tests and the Bikini tests of 1954 together contri uted about 5 per cent of the total sr90 activity. sr90 on the island is being maintained at an essentiaily constant level (decreasing only with physical decay), if the om vorous hermit crab can be considered an accurate index of biolo cally available strontium. However, the ratio of the strontiun in the crab skeleton to that in food items is not known. Judgin from the meager data presently available, the radiostrontium con tent of the crab skeleton is more than ten times that in plants on a wet weight basis and is more than land three times.that in soil on a dry weight basis. Muscle Isotopes with half lives greater than 20 years contributed nearly all of the activity in muscle tissue 35 days before Nectar test. C3si37, sp 7% x7? , and Celt, p pitt the accounted for 84, 10 and 1 per cent respectively, of the total activity in muscle tissue collected in February and November, 1955, and analyzed in January and March, 1956. Similar levels, 67, 19, and 1 per cent, were found in coconut crab muscle from Rongelap Atoll Table 14). (UWFL-43, In contrast to the exoskeleton, muscle tissue had a variable, though generally decreasing, level of long-lived isotopes throughout the post-Nectar collecting period (Fig. 6). Between 150 and 200 days post-Nectar, the total activity in muscle was due primarily to the long-lived isotopes as evidenced by the increased ecological half life. The level of total ac- tivity in muscle at 172 days (after Nectar) is. one-sixth the pretest level, while the level of long-lived isotopes at that oF 3 a AN oo