A study was undertaken to evaluate the anaerobic digestion of total raw olive-oil wastewater in a two-stage pilot-plant with reactors connected in series. Two different types of anaerobic digesters were used, an upflow type and a fixed-bed type. The main pupose of the study was to evaluate the performance of an up-flow-type reactor working in series with a fixed-bed-type reactor; which was used as a complementary treatment. The pilot-plant system operated in the mesophillic range (35+/-1 degrees C) during approximately 390 days, and with organic loading levels that ranged between 2.8 and 12.7 g COD/l.day. Concentrated aqueous ammonia was added to the total raw wastewater to adjust the CIN ratio to the optimum value of 20/1, and this also achieved stabilisation of the pH values in the digesters within a range about neutrality In a series of seven consecutive experiments, for the first stage (upflow digester) optimum values of specific biogas production rate stabilised at a value of 2.1 litres/litre digester.day with a very satisfactory COD reduction of 83% (with a volumetric load of 11 g COD/l.day). For the second stage (fixed-bed digester), the biogas production rate stabilised at a value of 0.22 litres/litre digester.day with a COD reduction of 8% (with a volumetric load of 0.19g COD/l.d). According to the results reported in the literature for total raw wastewater; the performance of the applied system is amongst the best for biogas production, COD reduction and loading rate reported so far; especially for the up-flow digester Phenols were greatly reduced during the anaerobic digestion process in both digesters, with a concentration reduction which reached 75% in the up-flow digester; with the use of the second stage (fixed-bed reactor) a further reduction of 45% was obtained. With the above encouraging results we may suggest the employment of the up-flow type digester as an economical and effective treatment for significantly reducing the organic load of total raw wastewater More satisfactory results might be expected from the use of a fixed-bed-type digester connected in series with a previous one as a second treatment stage. Copyright (C) 1996 Elsevier Science Ltd.