Incandescent lamp illumination enhanced methane production from a thermophilic anaerobic digestion reactor (55 degrees C) supplied with glucose. After 10 days of operation, the volume of methane produced from light reactors was approximately 2.5 times higher than that from dark reactors. A comparison of the carbon balance between light and dark conditions showed that methane produced from hydrogen and carbon dioxide in the light reactors was higher than that from the dark reactors. When hydrogen or acetate was fed into the reactors, methane production with added hydrogen was faster and higher under light conditions than under dark conditions. The use of blue light-emitting diodes also enhanced methane production over that under dark conditions. The 16S rRNA gene copy numbers for Methanothermobacter spp. in the light reactor and in the dark reactor were at the same level. The copy number for Methanosarcina spp. in the light reactors was approximately double than that in the dark reactors. These results suggest that blue light enhances the methanogenic activity of hydrogenotrophic methanogens.