A continuous laboratory-scale gas-lift reactor was used to investigate the influence of sludge retention time (SRT) on the competition for hydrogen between sulphate reducing bacteria (SRB), methanogenic archaea (MA) and homoacetogenic bacteria (HB). The reactor (30 degreesC; pH 7) was inoculated with anaerobic sludge and fed with H-2/CO2 and sulphate. The reactor was initially operated at a SRT of 2.6 days and loaded with hydrogen in excess while sulphate was limiting (H-2/SO4 ratio = 12.6). Sulphate reduction, methanogenesis and homoacetogenesis occurred simultaneously. The sulphide formation rate attained a stable level of approximately 3 gCOD l(-1) day(-1); the methane and acetate formation rates increased from 2.4 and 0.5 gCOD l(-1) day(-1) to 3.6 and 3.1 gCOD l(-1) day(-1) in 9 days, respectively. When sulphate was in excess (H-2/SO4 ratio = 1.9-4), SRB outcompeted MA and HB for H-2, independently of the applied SRT (2.6 and 19.8 days). However, the change from sulphate limitation to excess of sulphate resulted in a rapid drop of methanogenic activity at a SRT of 2.6 days: methane production fell from 3.6 to 0.2 gCOD l(-1) day(-1) in 5 days. At a SRT of 19.8 days, MA were outcompeted much more slowly: the methane production decreased from 1.5 to 0.1 gCOD l(-1) day(-1) in 40 days. Thus, elevated SRTs delay the outcompetition of MA, without affecting the final outcome of the competition. (C) 2003 Elsevier Ltd. All rights reserved.