This work aims to study effects of increasing wastewater loading rates (WLRs) on the performance of an up-flow anaerobic fixed bed bioreactor and on the dynamics of the bacterial community of the sludge using polymerase chain reaction-single stranded conformation polymorphism (PCR-SSCP) methods. The analysis showed that WLRs variations influence the bacterial community structure and affect the bioreactor performance. For WLRs of 0.34 g l-1 d-1 to 0.85 g l-1 d-1, the bioreactor showed a high performance and maintained highest colour and chemical oxygen demand (COD) removal yields with average values of 95% and 90%, respectively. The molecular fingerprint revealed a positive correlation between the diversity and the bioreactor performance. Increasing the WLR to 1.7 g l-1 d-1 affected significantly the bioreactor performance, the colour and COD removal efficiencies dropped to average values of 75% and 70%, respectively and bacterial and archaeal communities' profiles changed. The gas production rates increased when WLR increased. The highest value of 0.68 l l-1 d-1 was obtained at an hydraulic retention time (HRT) of 1 day. The use of molecular and microbiological methods to recover bacterial populations involved in this anaerobic process showed that fermentative (Clostridium spp.) and sulphate-reducing bacteria (SRB) (Desulfovibrio spp.) were the prominent members of the bioreactor bacterial community.