The performance of an upflow anaerobic sludge blanket (UASB) reactor operated at ambient temperature (20.9-25.2°C) was analysed for the treatment of slaughterhouse wastewater previously pre-treated for solid separation. The experimental work was carried out in a reactor with 15 L effective volume. Four organic loads were applied and the process performance was evaluated. The COD removal rate increased with the load rise from 4 to 15 kg COD.m(-3).d(-1). Removal efficiencies of 90% were obtained with a load of 15 kg COD.m(-3).d(-1). The entrapment of suspended solids in the sludge blanket was greater in proportion during the first two stages due to the low upflow velocities used when loads of 4 and 7 kg COD.m(-3).d(-1) were evaluated. This phenomenon did not affect the structure of the biological grains or their methanogenic activity. More than 50% of the organic nitrogen was degraded, causing a 3% increase of ammonia concentration. The concentrations of the volatile fatty acids were not high and the wastewater alkalinity was enough to prevent acidification. The yield coefficient of methane production increased with the load rise, reaching 0.266 m(3)/kg COD(removed) at 15 kg COD.m(-3).d(-1) organic load. The UASB reactor is a good option for the biological treatment of pre-treated slaughterhouse wastewater. However, additional treatment is required in order to accomplish the water quality requirements in discharges to water bodies.