We consider the problem of adaptive distribution of bits and power across the subcarriers in an OFDM radio-communication system operating in frequency-selective environments and propose a new joint adaptive bit and power allocation which ensures a significant increase in the data transmission rate for the given bit error probability. We present a scheme for practical implementation of the proposed algorithm. The results of mathematical simulation, which demonstrate the high efficiency of the proposed algorithm, are presented for the case of a multipath Rayleigh channel with an exponential multipath intensity profile. We study the influence of the Doppler variation in the frequency-selective channel in time on the efficiency of the proposed algorithm.