Abstract The design and Operation of neutrally buoyant floats that attempt to track the three-dimensional motion of water parcels in highly turbulent regions of the ocean, such as the upper mixed layer, are described. These floats differ from previous floats by combining high drag, a compressibility that nearly matches that of seawater, rapid (1 Hz) sampling, and short-range, high-precision acoustic tracking. Examples of float data are shown with the twin goals of demonstrating the utility of the floats and estimating the accuracy to which they are “Lagrangian.” The analysis indicates that these floats follow the motion of the surrounding water to better than 0.01 m s−1 under most circumstances. Both the net buoyancy of the float and its finite size contribute to the error. The float's buoyancy is controlled by making its compressibility very close to that of seawater, by making its drag large, by reducing air pockets and bubbles on the float, and by carefully controlling variations in the float's mass an...