The microbial community of a mesophilic lab-scale upflow anaerobic sludge blanket (UASB) reactor treating vitamin C biosynthesis wastewater at gradually elevated organic loading rates (OLRs) was characterized using 16S rDNA-based polymerase chain reaction-DGGE (denatured gradient gel electrophoresis) analysis. The DGGE fingerprints suggested that the elevated OLRs did not cause any significant changes in the microbial community. The predominant bacterial bands were affiliated with the Firmicutes (Clostridiales, four bands), Proteobacteria (Deltaproteobacteria, six bands), Bacteroidetes, and Synergistetes, respectively. All the archaeal bands were very similar to already known methanogenic species: Methanobacterium formicicum (two bands), Methanomethylovorans hollandica (one band) and Methanosaeta concilli (two bands), which belonged to the divisions Methanobacteria and Methanomicrobia, respectively.