PREFACE This is a record of experimental data required following Shot 5 and Shot 6 of Operation Castle together with a careful re-evaluation of its significance. ’ A preliminary report was put together under great pressure just following Operation Castle (and circulated in limited numbers as ITR-935, May 1954). There had been no time for thorough consultation between the collaborating organizations; certain computations were still not completed; in fact, the final calibration of gamma instruments had not been completed because the instruments themselves disappeared for several weeks in the course of transportation. The evaluation of the direct gamma measurements and oceanographic measurements was carried out at Scripps Institution of Oceanography; the analysis of water samples and their evaluation has been carri d out at U. S. Naval Radiological Defense Laboratory. Attempts have been made to integrate the effects at each institution. The authors wish to accord special acknowledgment to Feenan D. Jennings of Scripps Institution of Oceanography for his outstanding contributions during the cruise and in the analyses, and to John D. Isaacs and Roger R. Revelle of the sameinstitution for their useful guidance during the planning phases prior to Shot 5. Appreciation is expressed to R. L. Stetson and W. B. Lane, of U. S. Naval Radio~ logical Defense Laboratory, who assisted in plaming; to D. McDonald, U. S. Naval Radiological Defense Laboratory, who participated in the Shot 5 survey; and to many individuals of the NRDL and SIO staff who assisted in this study. Mrs. Suzanne Volkmann of the SIO staff has contributed extensively during the past year to the analyses and editing of the SIO contributions. The facilities and experience of the U. S. Bureau of Standards were made available to SIO for the calibration of the gamma devices. The authors are grateful to L. S. Taylor, Harold Wyckoff and S. W. Smith of that institution. Finally, the success of this experiment owes muchto the assistance of many individuals of Joint Task Force Seven for providing cquipment and a generous amount of personal time and good will.