additional fuel at some point during the 3,000-mile cruise. The proposed track finally was laid out on a grid (Fig. 7) providing distances of 45 miles betweenstations. Track lines running northeast and southwest, although calling for longer runs between stations than on north-south or east-west tracks, were used (a) to give continuous water monitoring readings that were only 32 miles apart, and (b) to put the operations at sea on headings that would minimize the ship's roll. The Actual Track: From the point of the initial drop to the return to Eniwetok for fuel on Jume 16, the WALTON followed rather closely the track that had been laid out in advance. During the second half of the cruise, however, it was necessary to modify the track to avoid possible contamination of the ship by tests scheduled in the area and to provide, in the vicinity of Bikini, a more complete coverage of the area contaminated by the drift of radioactive materials. The track actually accom- plished by the WALTON is shownin Figure 8, COLLECTION AND PREPARATION OF SAMPLES Plankton, water, and fish samples and continuous measure- ments of the activity in the surface water were taken. Plankton Samples and Water Temperatures: Plankton samples were obtained by twenty-minute oblique tows from about 200 meters of depth using a l-meter Michael Sars type net of No. 6 mesh (Fig. 3). No. 6 mesh is equivalent to seventy-four meshes per inch with apertures of 0.24 mm. This mesh retains some microplankton as well as larger forms. Organisms longer than 5 a fy ht