The biodegradation of BTEX compounds (benzene, toluene, ethyl benzene, and o,m, p-xylenes) by a mixed culture obtained from the formation of water, produced from the petroleum wells of the Turkish Petroleum Corporation (TPAO) in the Adiyaman region (southeast Turkey), and the effect of biomass concentration on the biodegradation rate of BTEX compounds, both alone and as a mixture, were investigated. The mixed culture, identified as Pseudomanas stutzeri and Vibrio mimicus, was grown on a brain heart infusion enriched medium at 30degreesC before its use in the biodegradation experiments. The biodegradation experiments were carried out using nonadapted, benzene-adapted, and toluene-adapted microorganisms. It was found that the mixed culture obtained degraded all BTEX compounds, both alone and as a mixture, and the overall specific biodegradation rates of BTEX compounds individually were higher with toluene-adapted microorganisms (both benzene and toluene: 4.27 mg/g biomass-day) than with the nonadapted (benzene: 0.096; toluene: 1.355 mg/g biomass-day) and benzene-adapted microorganisms (benzene: 3.4; toluene: 1.6 mg/g biomass-day). Similar results were obtained for BTEX compounds in a mixture. When the initial biomass concentration increased from 0.42 g/L to 2.34 g/L, the time required for complete biodegradation of both benzene and toluene decreased from 3 to 2 days with toluene-adapted microorganisms.