Two anaerobic filters, one mesophilic (35 degrees C) and one thermophilic (55 degrees C), were operated with a starch-based feed at a series of organic loading rates. At loading rates up to 8.3 kg COD m(-3) d(-1), there was no difference in the performance of the two types of reactor, measured in terms of the removal of soluble COD (SCOD) and gas production. At the higher loading rates of 12.4 and 17 kg COD m(-3) d(-1), the thermophilic filter gave the better performance; a SCOD removal of 93% compared to 78% at the former loading rate and one of 88% compared to 55% at the latter one. The daily methane production from the mesophilic digester was also lower; 3.19 ld(-1) compared to 4.98 ld(-1) for the thermophilic digester at the loading rate of 12.4 kg COD m(-3) d(-1) and 2.24 ld(-1) compared to 6.18 ld(-1) at the loading rate of 17 kg COD m-3 d(-1). The volatile fatty acid concentration in the effluent from the mesophilic filter also increased from <50 mg acetate 1(-1) to 254 and 464 mg acetate 1(-1) at the two higher loading rates.