The aim of this work was to study the performance of anaerobic fixed-film reactors with non-random support, for poultry slaughterhouse wastewater pre-treatment, including the influence of operating conditions. The work was carried out with two labscale reactors, one upflow and the other downflow, both equipped with Vertical corrugated PVC tubes as support and a recirculation circuit. Both reactors were operated at 35 degrees C. COD removal efficiencies ranging from 85% to 95% were achieved for organic loading rates of 8 kg COD m(-3) d(-1), while the highest organic loading rates (35 kg COD m(-3) d(-1)) led to efiiciencies of 55-75%. The reactors did not show destabilization after 12 h shock loads of 50 kg COD m(-3) d(-1). Reactor stability was easily achieved under intermittent operation, with weekend breaks, after which the reactors rapidly returned to their optimal performance. The influences of the hydraulic retention time, temperature, the recirculation ratio and flow direction were also studied.