In this study, it was aimed to investigate the effects of increasing sludge retention times (SRTs, 5, 10, 25, and 40 d) on dissolved chemical oxygen demand (CODdis), polycyclic aromatic hydrocarbons (PAHs) and acute toxicity removals in an aerobic completely stirred tank (CSTR) reactor system treating raw petrochemical industry wastewater. The reactors were operated with and without rahmnolipid biosurfactant. In both conditions the effluent quality of the CSTR reactor improved with increasing SRT. Addition of 15 mg/l rhamnolipid increased significantly both CODdis and PAH yields. For maximum COD and PAH removals (86 and 86%) the optimum sludge age was found to be 25 d. The effective PAHs concentration affecting half of the Daphnia magna organisms (EC50 value) was reduced from 65.61 to 1.86 ng/ml at the end of the aerobic treatment at a SRT of 25 d. The EC50 value for COD was reduced from 240 to 33 mg/l after aerobic treatment. Toxicity removals which originated from PAHs and COD were 97 and 86%, respectively.