Olive-mill wastewater (OMW), an agro-industrial by-product from olive-oil milling (with the three-phase extraction method), was used experimentally as 5% (v/v) of the medium to culture Scenedesmus obliquus CCAP 276/3A. The characterization of the wastewater indicated a nitrogen deficiency. The highest specific growth rate of S. obliquus ${mu}_m=0.022;h^{-1}$ was found when the medium was maintained at a constant pH value of 7.0. The biomass productivity, $P_b$ was determined by the influence of pH on the speciation of the dissolved $CO_2$. The greatest elimination of $BOD_5$ occurred at extreme pH values. The protein and chlorophyll contents presented a maximum value close to pH = 7.0. The highest crude-protein content in the biomass was 25.6%. The polyunsaturated and essential fatty acids presented a clear trend to increase with the pH, at pH 9.0, representing 26.2 and 19.4%, respectively, of the total fattyacid content of the biomass.