A method and apparatus are provided for selecting a transmission mode based upon a length of at least a portion of a packet to be transmitted. A packet is transmitted in a multiple antenna communication system by selecting a transmission mode based on a length of at least a portion of the packet. The transmission mode can indicate a number of antennas or date rate (or both) to be used for the transmission. A transmission mode can also be selected based on, for example, (i) a mode selection table that records an expected transmission time for a packet for each supported transmission mode or (ii) one or more packet length thresholds each having a corresponding transmission mode. ACK (acknowledgment) packets can be transmitted using a predefined transmission mode, such as a SISO mode (or a lower order MIMO mode than the original packet), or based on a predefined relationship between the transmission mode of the original packet and the corresponding transmission mode that should be used to acknowledge the packet.