10 radioactivity in the plankton samples. The rate of movement can only be approximated because the origin of the fallout at any one sampling station is not known accurately in respect to either time or area. The origin may be any of many detonations that occur during the testing period which lasts for several months and for which the area of the original local fallout is only known within several hundred miles. In 1956, fallout, as indicated by the radioactivity of the plankton samples, was 800 miles west of Eniwetok one month after the end of the Redwing series but it is to be remembered that this fallout could have originated several months earlier at the beginning of the Redwing series. In 1958 at a comparable time in respect to the end of the test series fallout was detectable in the plankton 1100 miles west of Eniwetok. The best estimate of the westward rate of advance is about ten miles per day or slightly less. In 1958 a surface drogue placed a few miles west of Eniwetok moved 51 miles in 71 hours or about 17 miles per day. This rate of movement was for a short distance and is a measure of the surface current which would be expected to be faster than the rate of advance of fallout as measured by the radioactivity in plankton. Radiochemical separations by ion exchange and precipitation techniques and gamma spectrum analyses were used to determine the radioisotopes present in the plankton samples.- For the Redwing samples fission products, mainly Zr? -Nb?? and cel44_py 144