The treatability of dilute wastewater by a UASB reactor was evaluated. After being seeded with digested sewage sludge, a 110-l reactor was operated at 25 degrees C under continuous feeding with synthetic wastewater having a COD of 300 mg/l for over 700 d. The hydraulic retention time (HRT) was successfully decreased in steps to 9 h (0.73 kg COD/m(3)/d). Granules began to be observed after the reactor had been in operation for 9 months. At an HRT of 9 h, the following results were obtained : soluble and total COD removal, 81+/-5% and 73+/-7%, respectively; methane production, 212 Nl/kg COD removed and 153 Nl/kg COD added; substrate conversion, 0.24 kg COD removed/kg VSS/d.