A 96 m(3) UASB reactor was operated for 2.5 years under different conditions to assess the feasibility of treating strong sewage (CODtot = 1531 mg/l) at ambient temperatures, with averages of 18 and 25 degreesC for winter and summer respectively. During the first year, the reactor was operated as a two-stage system at OLRs in the range of 3.6-5.0 kg COD/m(3) d for the first stage and 2.9-4.6 kg COD/m(3) d for the second stage. The results of the first stage showed average removals of 51% and 60% for CODtot and CODss respectively without significant effect of temperature. The second stage reactor was unstable. The temperature affected sludge stabilization. During the second year, the first stage was operated as a single-stage UASB reactor at half of the previous loading rates. The results showed an average removal efficiency of 62% for CODtot during summer, while it dropped to 51% during wintertime. However, the effluent suspended solids were stabilized with VSS/TSS ratio around 0.50 all over the year. The sludge in the single-stage reactor was well-stabilized and exerted an excellent settlability. During the last three months of research, sludge was discharged regularly from the single-stage UASB reactor. The results showed no significant improvement in the performance in terms of CODtot. Based on the results of the experiment, a single-stage UASB reactor operated at relatively long HRT is preferred above two-stage system at the Jordanian conditions. (C) 2004 Elsevier Ltd. All rights reserved.