The effect of changes in the numbers and the composition of the microbial population on the performance of a pilot-scale crossflow ultrafiltration membrane anaerobic contact reactor (CUMAR) system was studied during both start-up and steady-state operation. An initial epifluorescent microscopic examination of the seed sludge taken from a digester of a municipal wastewater treatment plant revealed that the most dominant group was Methanococcus followed by Methanosarcina, short rods, medium rods, filaments and long rods. Changes in the composition of the dominant methanogenic species were observed throughout the operation. Short rod species were found to be the most dominant group at the end of the study followed by medium rods, Methanococcus, Methanosarcina, filaments and long rods. The numbers of both methanogens and non-methanogens were found to have increased 100% and 50% respectively in the digester at the end of the study. Although the proportion of methanogens in total bacteria in the digester gradually increased from 6.8% to 9.5% the numbers of viable methanogens sharply increased, i.e. 50 times, resulting in a 60 fold increase in the metabolic activity of the methanogens. The maximum OLR applied was 28.5 kg COD/m(3).d, at which point COD removal efficiencies of approximately 99% in the permeate and over 97% in the digester were achieved showing that the changes in the numbers and the composition of dominant methanogenic species did not have adverse effect on the performance of the CUMAR system.