Bacterial and archaeal communities, as well as the digester   performances, were analyzed in order to evaluate the influence of  hydraulic retention times. A 40m³ pilot plant anaerobic thermophilic   bioreactor was used for the experiments and fed with poultry litter.  Chemical composition of feed, effluent and biogas were monitored  throughout the experiment. Mass and energy balances were used to  evaluate the performance of the biodigester. Samples of 
 microbial  community DNA were collected each week during the experiment.  The  relative abundances of dominant microbial populations were measured  by terminal restriction fragment length polymorphism (T-RFLP) using  PCR primers targeting the bacteria and archaea.  Results indicate that loading rate plays an important role in biogas production and  reduction of COD and BOD.  These changes were associated with variable  microbial population dynamics in the bioreactor.  T-RFLP profiles of the bacteria indicate that 4 ribotypes dominated the profile throughout the experiment.  The abundance of 2 of the 4  ribotypes changed during the course of the experiment  while 2  ribotypes remained relatively constant.  Archaeal T-RFLP profiles revealed 1 dominant ribotype which does not appear to change in  abundance.  The results of this experiment, both power input and biodigester performance, indicate that the 13d HRT was optimal for the anaerobic digestion of poultry litter in the experimental conditions  evaluated here.