Methanogenic activity in thermophilic, anaerobic reactors was determined by comparing the amount of methane generated in single- and two-stage systems with the size of the methanogenic population, as determined by microscopy. The methanogenic activities were 2.71 x 10(-9) ml methane cell(-1) d(-1) and 1.10 x 10(-9) ml methane cell(-1) d(-1) for 10 and 4 days of the hydraulic retention time (HRT), in the single-stage system. In the two-stage system, 7.49 x 10(-9) ml methane cell(-1) d(-1) in the acidogenic reactor and 1.56 x 10(-9) ml methane cell(-1) d(-1) in the methanogenic reactor for 4 days of the HRT. A high correlation was evident between the methane production and methanogenic population [0.1354 ln(x) -2.1375](R-2 0.8619).