This thesis studies the performance of the Anaerobic Fluidized Bed Reactor (AFBR) in treating diesel fuel contaminated wastewaters. The AFBR is a semi-cyclindrical fluidized bed, with a capacity of 300 L and height of 2.90 m with sampling ports along the column length. Granular activated carbon (12-20 mesh) was used as the medium to immobilize biomass. Diesel fuel was the sole source of carbon for microorganisms. The system's COD removal capability and diesel fuel removal efficiency were measured at 100 mg/L, 200 mg/L, and 300 mg/L influent diesel fuel in the reactor. Hydraulic Residence Time (HRT) varied for each set of experiments from 96 to 6 hours. The system achieved diesel removal efficiency of more than 84.1% for 300 mg/L influent diesel concentration for the maximum flowrate treated (1200 L/d) at a minimum HRT of 6 hours. This investigation confirms that anaerobic degradation of diesel contaminated water can be carried out very effectively in the AFBR.