Many techniques for constructing unitary space- time constellations have been proposed. To minimize bit-error rate (BER) in a wireless communication system, constellations constructed using these techniques should be given a Gray- code type bit assignment, where symbols which are close in signal space have bit assignments which have small Hamming distance. To the authors' knowledge, no efficient general strategy for making this bit assignment has been suggested. This work proposes a prioritized distance (PD) algorithm for making this assignment in an optimal manner by minimizing the probability of bit error union bound. The algorithm can be used on constellations constructed using any technique. Simulation results show this algorithm significantly outperforms random searches and achieves near globally optimum results with moderate complexity.