A laboratory-scale anaerobic filter (AF) and a fluidized-bed reactor (FBR) were compared for their ability to treat palm oil mill effluent (POME). The reactors were operated at mesophilic temperature (35 degrees C). Reactor performance was assessed by measuring chemical oxygen demand (COD), volatile fatty acids (VFA), gas composition and gas production. Sand of 0.3-0.5 mm diameter was used as a biomass support medium in the FBR, and clay rings of 2-5 mm diameter were used in the AF. Start-up of the AF was achieved with 1.5-4.5-day residence times and FBR residence times were maintained at 6 h. After acclimatization, COD removals higher than 90% were reached in both reactors at 6 h residence time, equivalent to a loading of 20 g COD/litre/day. Ar higher loadings the FBR gave a better performance; even at 40 g COD/litre/day, with 6-h residence times, 78% COD was degraded. The AF could not be operated above 20 g COD/litre/day without clogging. The AF and FBR performed similar at reactor concentrations up to 1 g COD/litre, while above 2.2 g COD/litre the AF showed a maximum removal rate of 17.0 g COD/litre/day compared to 31.2 g COD/litre/day for the FBR. These differences were probably due to diffusion limitations and a less active biomass in the AF. In both reactors the gas production rate and the levels of organic acids increased in response to a higher feed concentration in less than one day and this could be explained by substrate limitation. Other responses were slower with the microbial culture adapting over periods of 6-10 days; these were apparently growth related. A doubling in loading always resulted in a large increase in organic acids, especially acetic and propionic, as well as increasing the proportion of CO2 in the gas.