Aims: The suitability of genetic fingerprinting to study the microbiological basis of anaerobic bioreactor failure is investigated.Methods and Results: Two laboratory-scale anaerobic expanded granular sludge bed bioreactors, R1 and R2, were used for the mesophilic (37 degrees C) treatment of high-strength [10 g chemical oxygen demand (COD) 1(-1)] synthetic industrial-like wastewater over a 100-day trial period. A successful start up was achieved by both bioreactors with COD removal over 90%. Both reactors were operated under identical parameters; however, increased organic loading during the trial induced a reduction in the COD removal of R1, while R2 maintained satisfactory performance (COD removal > 90%) throughout the experiment. Specific methanogenic activity measurements of biomass from both reactors indicated that the main route of methane production was hydrogenotrophic methanogenesis. Terminal restriction fragment length polymorphism (TRFLP) analysis was applied to the characterization of microbial community dynamics within the system during the trial. The principal differences between the two consortia analysed included an increased abundance of Thiovulum- and Methanococcus-like organisms and uncultured Crenarchaeota in R1.Conclusions: The results indicated that there was a microbiological basis for the deviation, in terms of operational performance, of R1 and R2.Significance and Impact of the Study: High-throughput fingerprinting techniques, such as TRFLP, have been demonstrated as practically relevant for biomonitoring of anaerobic reactor communities.