ABSTRACT The trend in beta radioactivity as measured with methane flow counters over a period of about two years is shown, starting with the 1954 Castle series of nuclear detonations, up to but not including the series of 1956. The results are presented as graphs each showing the logarithm of the radioactivity of an organism or of a particular tissue of an organism, related to the logarithm of the time after the date of detonation, when nearly all of the radioactivity was assumed to have originated. Invertebrates are considered in greatest detail, and other organisms and materials are included for comparison: island soil, beach sand, sea water, plankton, algae, land plants, reef fish, birds, and rats. It is proposed for most organisms studied that after a period varying with the organism up to two to four weeks following detonation, a maximum level of radioactivity in the field samples collected is attained, followed by a decline approaching linearity on log-log plots with slopes over the major portion of the two-year period that can be represented as the negative exponent of the time after detonation. These decline slopes varied greatly with different localities and organisms, reaching 4& maximum of > 3. A few decay rates of individual samples of each organism or material are included for comparison, and these generally were equal to, or less steep than, the declines, suggesting that for some organisms or tissues, the level of radioactivity in the environment decreases more rapidly than oan be accounted for solely by physical decay while for others the rate of decline can be accounted for solely by the rate of physical decay. Dilution by natural water currents and rain is pre- sumed to account for the many cases of more rapid decline than decay.