In a method for simultaneously communicating with multiple communication devices in a wireless local area network a first communication device receives a plurality of uplink data units simultaneously transmitted by multiple second communication devices. The first communication device generates an acknowledgement data unit to acknowledge receipt of the multiple data units simultaneously transmitted by multiple second communication devices. The acknowledgement data unit includes (i) an indication that indicates that the acknowledgement data unit is intended for multiple second communication devices and (ii) respective acknowledgement information for the multiple second communication devices. The acknowledgement data unit is transmitted from the first communication device to the multiple second communication devices.