-1bretios for other biological samples from within Eniwetok and Bikini Atolls, reported in an earlier paper, are given (16). Co? and Co60 were present at Bikini Atoll in a 1:1 ratio; at Eniwetok in a 5.3:1 ratio. A similar difference in levels of the two cobalt isotopes also occurred in the plankton taken in the open sea in the vicinity of the Pacific Proving Ground. Thus, at stations 57, 58, 59, approximately 200 miles northwest of Bikini Atoll, the ratio Co57/co®° is 4.6:1.0 and the value 150 miles northwest of Eniwetok Atoll is 7.6:1.0. The ratio of Co>?/co®° in an area 300 miles wide north to south and 200 to 500 miles west of Eniwetok was constant at a ratio of ap- proximately 11:1. The ratios of CoD? /zn®5 for the plankton stations are shown in Figure 4b. The values for the five plankton samples on which ion-exchange separations were made are shown crosshatched; the remeining CodT /zn®5 ratios for plankton were calculated from the gamma spectra on total samples. The ratios for Eniwetok and Bikini samples were taken from ionexchange separation data in an earlier report (16). POE ARCHIVES The ratio of Co57/zn©5 at both atolls was 0.06:1.0, indicating a relatively high level of 2n©5 with respect to Col, The same condition prevailed at stations 3, 4, 5, and 6, 200 miles south and west of Eniwetok Atoll. However, the ratio north of Eniwetok and Bikini along the northern boundary of the restricted zone was slightly higher (0.2:2.0) than that of the atolls. The center of high Co?’ /zn©5 ratio (5.5:1.0) was 19