The low yields of methane production from olive mill wastewaters (OMW) which occur at high chemical oxygen demand (COD) concentration are caused by the presence of inhibitory compounds in this industrial effluent. These include tannins, phenolic compounds and oils which are toxic to methanogenic bacteria. In contrast, Aspergillus niger will grow on undiluted OMW, yielding reductions in COD and phenolic compounds of 61 and 58% respectively. The treated OMW is detoxified for methanogens, which will now grow easily on this treated material. Such aerobic pretreatment offers a novel approach to the degradation of OMW.