Abstract-In CSMA networks, there is a significant overhead associated with each packet including header overhead and contention overhead. The high overhead problem is exacerbated because some of these overheads are required to be at the lowest data rate to ensure that all contending nodes can compete fairly. For applications where packets are small, such as Voice over IP (VOIP), this means that a majority of the available transmission time is wasted on overhead. Packet aggregation is one of the techniques that has been proposed to amortize the per-transmission overhead over multiple aggregated packets. However, existing heuristics are limited, often not considering multi-rate wireless MAC, or operation in a WLAN environment. In this paper, we first formulate the problem of optimal aggregation for a multi-rate CSMA MAC protocol and show that it is NP-Hard. We then propose two heuristics that solve the aggregation problem for multi-rate WLANs. The first, which we call Data Rate based Aggregation protocol (DRA), divides packets in the MAC queue into groups based on the data rate they are to be transmitted at. The algorithm aggregates packets in the same group and broadcasts the aggregated frame at the data rate of that group. Empirically, DRA achieves several fold increase in throughput compared to basic aggregation. DRA also achieves up to a 200% increase in the number of VoIP calls supported by a single 802.11g AP compared to using state of the art aggregation protocols. The second heuristic, which we call Data Rate based Aggregation with Selective Demotion (DRA-SD), enables cross data rate aggregation. Through preliminary evaluation, we show that selectively demoting packets can further improve performance.