A first communication device receives a physical layer (PHY) data unit from a second communication device. The PHY data unit includes multiple medium access control (MAC) data units aggregated in an aggregate MAC data unit. The first communication device generates an acknowledgment data unit to acknowledge receipt of the multiple MAC data units. The acknowledgment data unit includes (i) a length indication that indicates a length of an acknowledgement field, and (ii) the acknowledgment field of the indicated length. The acknowledgement field includes respective acknowledgement information for the multiple MAC data units. The first communication device transmits the acknowledgment data unit to the second communication device.