Anaerobic treatment of olive oil mill wastes, namely black water and prina, was investigated in batch reactors. Biochemical methane potential (BMP) tests were conducted to determine the anaerobic biodegradability of black water and/or prina. With these BMP tests the biodegradability of olive mill wastes (OMWs) at different initial chemical oxygen demand (COD) concentrations and corresponding methane gas productions were investigated. Furthermore, a screening study was performed to determine the most important nutrients for the anaerobic digestion of black water. The results indicated that OMWW could be treated anaerobically with high efficiencies (85.4-93.4%) and treatment of 1 l olive mill waste waters (OMWW) by anaerobic methods resulted in production of 57.1±1.5 l of methane gas. Anaerobic treatment of the olive mill residual solids (OMRS) alone was poor; however, when OMRS was mixed with OMWW in certain ratios, OMRS could be treated efficiently under anaerobic conditions. Anaerobic cultures needed an adaptation period of 15?25 days for treatment of OMRS with and without OMWW.