-li- bace izes basis of the average amount of Sr?° in the 44 specimens collected during the. first 50 days following Nectar, less the amount present before Nectar. The relative radioactivity of the two fsotopes was calculated from their specific activities. A theo- retical decay curve was then calculated for the combined grB9 , gr7°+ x9? contributed by the Nectar test and the gr99, y70 re- sidual from prior tests. Figures 4 and 5 show the actual values superimposed on this theoretical curve. Although there were no specific radiochemical determinations early in the period following Nectar it is reasonable to assume that the exoskeleton has a high degree of selectivity for strontium and that equilibrium must be reached within a few days at most. ‘The assumptions are further supported by decay curves which approach the theoretical curve (Fig. 4). The relatively low levels of activity at 145 days postNectar are 4& reflection of a change in ratio of ash weight to wet weight; Figure 4 represents the data on an ash weight basis. The change in ratio may be associated with molting, but observations were not made at frequent enough intervals to confirm or deny such an association. Contributions of radiostrontium to the crab skeleton at Belle Island from past tests at Eniwetok and Bikini are represented in Figure 5. The pre-Mike level 1s an approximation since it is based on @ single specimen and there was, unfortunately, no biological survey during the 1950 tests. were derived by the method outlined above. The pre-Nectar curves The Mike test contrib- uted about twice as much activity as the Nectar test; fallout from oe