A first communication device receives plurality of uplink data units simultaneously transmitted by multiple second communication devices, and generates an acknowledgement data unit to acknowledge receipt of the plurality of uplink data units. Each of at least some of a plurality of acknowledgment fields includes: a first field that includes an indicator of a corresponding one of the second communication devices, a second field that indicates whether the acknowledgment field corresponds to i) an acknowledgment corresponding to a single data unit, or ii) a block acknowledgment corresponding to multiple data units, wherein when the second field that indicates a block acknowledgment, the acknowledgment field further includes a third field having a bitmap that indicates which data units in the group are being acknowledged, and wherein when the second field that indicates acknowledgment of a single data unit, the acknowledgment field does not include the third field.