We present an orthogonal frequency-division multiplexing (OFDM) system with bit-interleaved space-frequency coded modulation for frequency-selective fading channels employing multiple transmit and receive antennas. OFDM is used to transform a frequency-selective fading channel into multiple flat fading channels, and space-frequency trellis coding combined with bit-interleaving is used to exploit space and frequency diversity. The performance of such an approach is evaluated by analytical bounds and simulation. The proposed scheme is an efficient method to employ multiple transmit antennas to increase the data rate and, at the same time, improve the performance by means of increased diversity obtained with bit-interleaved coded modulation and multiple receive antennas. A block precoding method is proposed to further improve the diversity order. An efficient sphere decoding algorithm is also developed for reducing the decoding complexity of the proposed scheme.