During the preliminary survey at Bikini, the state of turbulence in the lagoon was determined from three types of measurements: simultaneous measurements of surface current and wind speed, measurements of diurnal temperature fluctuations at various depths, and measurements of the rate of mixing of the oxygen‐rich reef water with the body of the lagoon water. These measurements lead to reasonably consistent values for the coefficient of vertical diffusion. The coefficient of horizontal diffusion was estimated on the basis of past observations and of measurements of the lateral spread of dye markers. Its estimated value compares favorably to a value computed from the lateral spread of radioactive material deposited during the underwater atom‐bomb bu