- 46 that the nets were of nylon rather than silk and the mesh was slightly more open in the "fine"net. (For the "fine" net, mesh size was 157 per inch as compared to 174 per inch when silk was used. ) The radioactivity of the plankton samples from the six collections since March 1954 is summarized in Table 12. It will be noted that the present level of activity in Rongelap lagoon is less than 0.5 of a microcurie per kilogram of wet sample, that the radioactivity of the plankton off Rongelap Island continues to be less than that of the sample off Kabelle Island, and that the radioactivity of the plankton in Ailinginae lagoon was somewhat greater than the radioactivity of the plankton in Rongelap lagoon. The signi- ficantly greater activity of the Ailinginae plankton was unexpected. This greater value may be within the error that is typical of plankton samples; another explanation is the possibility that the exchange of ocean water and lagoon water is slower at Ailinginae than at Rongelap. The shape of the atolls would suggest this althcugh the relative rates of exchange are not known. Rongelap is roughly circular in shape with wide, deep passes in both the northeast and scuth while Ailinginae is rectangular in shape with its largest pass, in the southern part of the atoll, shallower than the Rongelap passes (Fig. 1). east. The wind-driven ocean currents are from the north-north- Water samples from off Rongelap Island and from Ailinginae lagoon do not indicate differences in activity, possibly because the counting techniques of the Laboratory are insensitive to very low counts. The rate of decline of radioactivity of Rongelap lagoon plankton was de- termined from the data in Table 12 by averaging all the values for any one