-28- The rate of decline of the radioactivity in almost all of the fish was greatest during the first 100 days, during which time more than 90 percent of the post-shot radioactivity was lost. The vartation in amounts of radioactivity in tissues from the same collection of fish, as measured by the coefficient of variation (v), may be due to several causes, prominent of which are the differences between different species of fish. The vari- ations definitely indicate that large samples of many species are necessary to obtain reliable information on the amount of radioactive materials present in fish populations.