During the anaerobic digestion of olive mill wastewater (OMW) prefermented by aerobic growth with Geotrichum candidum, a steady state was reached more quickly with the anaerobic contact process than with an anaerobic filter. The daily methane production and COD removal recorded with the anaerobic filter were greater than those obtained in the anaerobic contact reactor. The anaerobic filter yielded a biogas with a higher percentage of methane, and effluent with a lower volatile fatty acid and volatile solid (VS) content than the anaerobic contact reactor. The immobilizing of VS in an anaerobic filter fermenter is a means of reducing the inhibition of methanogenic bacteria by the residual phenolic compounds present in prefermented OMW. A yield of 0.18 and 0.34 litre methane/g COD added was obtained with the anaerobic contact and anaerobic filter reactors, respectively. Additional advantages of fixed film over contact fermenters include the elimination of mechanical mixing and sludge settling and return.