The effect of up velocity and influent COD concentration on the activity of sulphate-reducing bacteria (SRB) in UASB reactors is discussed. To study these effects, four UASB reactors were built and utilized in parallel. Examinations were carried out in two different concentrations of molasses (500 mg COD/1 and 1000 mg COD/1) and four different upward flow velocities. It was observed that at velocities greater than 1 m/h, SRB bacteria were easily washed out from the reactors due to lower density and lack of ability to form dense and firm granules. It was found that in low-strength wastewaters with a COD to sulphate ratio of 2, an upward velocity in the range of 1.5-2.5 m/h could be appropriate. At lower velocities, there would be significant numbers of SRB bacteria and so a toxicity risk to the system. At higher velocities, the COD removal efficiency may decrease due to lower hydraulic retention time in the system. It was also observed that increasing COD concentration improves COD removal efficiencies but has little effect on washing out of SRB. (c) 2004 Elsevier Ltd. All rights reserved.