The objective of this research was to evaluate the feasibility, the stability and the efficiency of a start-up at 20 degreesC and 25 degreesC of anaerobic sequencing batch reactors (ASBRs) treating slaughterhouse wastewater. Influent chemical oxygen demand (COD) and suspended solids concentrations averaged 7500 and 1700mgdm(-3), respectively. Reactor start-up was completed in 168 and 136 days at 20 degreesC, and 25 degreesC, respectively. The start-up process was stable at both temperatures, except for a short period at 20 degreesC, when effluent volatile fatty acid (VFA) concentrations increased fi om an average of 40 to 400mgdm(-3). Effluent quality varied throughout start-up, but in the last 25 days of the experiment, as the ASBRs were operated under organic loading rates of 2.25 +/- 0.21 and 2.86 +/- 0.24kgm(-3) d(-1) at 20 degreesC and 25 OC, respectively, total COD was reduced by 90.3% +/- 1.3%. Methanogenesis was not a limiting factor during start-up. At 20 degreesC, the limiting factor was the acidification of the soluble organics and, to a smaller extent, the reduction of propionic, isobutyric and isovaleric acids into lower VFAs. At 25 degreesC, the limiting factor was the hydrolysis of particulate organics. To minimize biomass loss during the start-up period, the organic loading rate should be increased only when 75 -80% of the COD fed has been transformed into methane within the design hydraulic retention time.