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In this correspondence, a joint optimization of spatio-temporal (S-T) power allocation to minimize the average bit error rate (BER) subject to an average power constraint is developed for beamforming orthogonal space-time block coded multiantenna systems over flat Rayleigh fading channels in the presence of imperfect channel state information at the transmitter (CSIT). Compared to the spatial-only (S-O) power allocation methods, the optimal S-T strategy reduces BER significantly. It can be used as a benchmark to evaluate the existing suboptimal S-T schemes. A necessary and sufficient condition, which can determine the number of eigenbeams being used for optimal S-T power allocation without computing power allocation, is derived. This result can greatly simplify the power allocation algorithm. Numerical results show that the proposed optimal S-T algorithm has not only the best BER performance, but also the lowest computational complexity in comparison with the existing S-T strategies.