Preliminary experiments to evaluate the behaviour of anaerobic fixed-bed reactors treating sugar-mill wastewater were carried out Four hydraulic retention times (0.5, 1.0, 2.0 and 4.0 days) at three levels of strength (raw wastewater, diluted 1:1 and diluted 1:2) were tested. Three glass columns of 1 m height and 6 litres total volume, packed with gravel of 3 cm average particle size, and bed porosity of 0.53 were employed It was found that organic matter removal increased with the hydraulic retention time and decreased with the influent concentration and such reactors might be used for treatment of sugar-mill wastewaters.