The performance of a pilot-scale (55-l) upflow sludge blanket (UASB) reactor treating municipal sewage without any supplementary heating was examined in a 200-day trial. The flow rate to the digester was kept constant, giving a hydraulic retention time of 7.6 h. Although there were some variations in the strength of the sewage, after the initial 18 days the loading rate was maintained at about 1.21 kg COD/m3 day. A pre-granulated sludge was used as the inoculum, but the start-up period was still long; about 90 days. The total volatile fatty acid:alkalinity ratio was initially high, indicating an element of instability in the digester. However, as more alkalinity was added to the incoming sewage, the ratio fell below the critical value. Once this had been achieved, the digestion process stabilized and achieved a COD removal efficiency averaging 85% and a specific gas yield of 0.34 m3 methane/kg COD removed.