The aim of this study was to identify the microorganisms present on the graphite cloth of the bioanode and biocathode of a Microbial Electrolysis Cell (MEC) using 16S ribosomal RNA gene cloning and sequencing. The results obtained indicated that the bioanode clones were related with the phyla: Firmicutes (15.3%), Proteobacteria (7.6%), Bacteroidetes (30.7%), and Ignavibacteriae (7.6%). Conversely, the biocathode clones were related with the phylum Proteobacteria (38.4%). The bioanode clones were related with species identified previously in MECs and Microbial Fuel Cells (MFCs). However, the biocathode clones were related with Rhodopseudomonas palustris, which have not been reported for hydrogen production by MEC. R. palustris probably should be involved into hydrogen production of 0.011 m 3 H 2 /m 3 cathode liquid volume per day with an applied voltage of 1 V.