In this study, specific methanogenic activity (SMA) test and fluorescence in situ hybridisation (FISH) were respectively used to determine acetoclastic methanogenic capacity, and composition and number of methanogenic and sulphate reducing bacterial (SRB) populations within a full scale anaerobic contact reactor treating a pulp and paper industry effluent. The sludge samples were collected from three different heights along the anaerobic reactor having a difficulty of completely stirring. Performance of the anaerobic reactor in terms of COD removal efficiency varied between 47 and 55% at organic loading rates in a range of 1.6-1.8kg COD m(-3) d(-1) and methane yield varied between 0.18 and 0.20 m(3)CH(4)kg CODrem-1. The anaerobic reactor was not operated for 2 weeks during the monitoring period. According to SMA test results, potential methane production rate was 276 mLCH(4) gVSS(-1) d(-1) before the off period of the reactor, however it decreased to 159 mL CH4 gVSS(-1) d(-1) after this period. SMA test and FISH results along the reactor height showed that the acetoclastic methanogenic activity of the sludge samples, the relative abundance of acetoclastic methanogens, hydrogenotrophic methanogens and acetate oxidising SRB, decreased as the reactor height increased, however the relative abundance of non-acetate oxidising SRB increased.