The directivity index Diof an underwater sound source usually is approximated because of the difficulty involved in performing the required power measurements and integrating over all angles. A simulation study conducted at the Underwater Sound Reference Division resulted in a model of a Dimeasuring system that minimizes the complexity of measuring the power profile about a sphere with accuracy to within ±0.1 dB. A system was assembled in accordance with the model and experimental measurements have been made in a shallow lake. The instrumentation and measurement techniques are presented in this paper. A complete three-dimensional power profile can be obtained in approximately 10 min running time with conventional hardware. Measured values of Diare in close agreement with theoretical calculations based on a perfect piston raditors, except for patterns with significant sidelobes.