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IEEE 802.11 WLAN service is broadening its service area rapidly. One of the major driving factors of WLANs is multimedia services such as VOD or HDTV delivered by multicast protocols. However, there have been little research on how to efficiently operate multicast protocols in IEEE 802.11 WLANs. The most widespread variants of IEEE 802.11 standards, 802.11 b and 802.11 a, are characterized by the rate adaptation mechanism depending on time-varying wireless link conditions. However the current IEEE 802.11 WLAN standard does not utilize the rate adaptation function for multicast transmissions. In this paper we propose a new rate adaptation mechanism for multimedia multicasting in IEEE 802.11 WLAN environments, called SARM (SNR-based Auto Rate for Multicast). SARM is based on the RBAR (Receiver Based Auto Rate) protocol. Therefore SARM adapts its transmission rate based on the SNR values of mobile nodes. In order to obtain SNR values, since there are no RTSCTS messages or MAC layer ACK messages in multicast transmissions, SARM needs an additional signalling method by which the AP can estimate channel quality (SNR). By adapting multicast transmission rate based on SNR, SARM guarantees the best quality of multimedia service depending on the mobile nodes’ link conditions. Moreover the proposed protocol minimizes the portion of wireless channel resource for multimedia multicasting, so that it can allocate more bandwidth to other unicast traffic.