Changes in the activity and bacterial populations of various trophic groups and the morphology of the sludge over time were investigated in a UASB reactor treating dilute wastewater. The specific methanogenic activity on day 400 was 6.9 (0.30 kg CH4-COD/kg VSS/d), 2.5 (0.36), 0.83 (0.03) and 4.4 (0.19) times as large as the activities of the seed sludge when acetate, H-2-CO2, propionate in the absence of sulfate, and propionate in the presence of sulfate were respectively used as test substrates. The viable cell numbers of trophic groups increased by 2 to 4 orders of magnitude for methanogens, 3 orders for acetogens, and 2 to 3 orders for sulfate-reducing bacteria as compared to the numbers of viable cells in the seed sludge. The anaerobic ciliate Metopus es incorporating methanogens was also observed.