An aerobic fluidized bed reactor (FBR) was operated for the removal of methyl tert-butyl (MBE) and benzene, toluene, ethylbenzene, and p-xylene (BTEX) from water. The reactor was seeded with a mixed culture adapted to MTBE. Granular activated carbon (GAC) was used as the biological attachment medium. Influent MTBE to the reactor was 7.8 mg/L MTBE, with a flow rate of 22.7 L/day, and an empty bed contact time of 1 hour. The acclimation period required was relatively short, about 30 days before reaching an average stable effluent concentration of 18.5 +/- 10 microg/L. BTEX was introduced to the feed at an equivalent chemical oxygen demand (COD) as the MTBE at day 225 and was biodegraded spontaneously with no apparent acclimation period required. The average influent of each of the four BTEX compounds was about 2 mg/L, and the range of the average effluent concentrations wae 1.4-2.2 microg/L. After achieving 180 days of stable performance with BTEX addition, the total low rate to the reactor was gradually increased by 20% increments to 160% of the original flow (36.4 L/day). Increases by 20% and 40% had no apparent effect on reactor performance, but increase by 60% required 30 days before effluent quality returned to previous values. Composition of the culture was monitored throughout operation of the reactor using denaturing gradient gel electrophoresis (DGGE). The culture consisted of Flavobacteria-Cytophaga and organisms with high similarity to the known MTBE degrader PM1.