An analytical formula is derived for the throughput of IEEE 802.11 WLAN in which stations may have different data rates or SNR. The throughput is significantly reduced from the presence of even one station with low data rate. This reduction is a consequence of the DCF mode which guarantees equal packet transmission probability to all stations. The throughput can be increased if stations with low data rates transmit shorter packets. The analysis is based on a semi-Markov model and the theoretical predictions are verified by simulation.