Methanogenesis was studied using stirred, bench-top fermenters of 3-1 working volume fed on a semi-continuous basis with waste obtained from cattle fed a high grain, finishing diet, Digestion was carried out at 40 and 60 degrees C. CH4 production was 11.8, 18.3, 61.9 and 84.5% higher in the thermophilic than the mesophilic digestor at the 3, 6, 9 and 12 g volatile solids (VS) l(-1) reactor volume loading rates, respectively. When compared on an energetic basis CH4 production was 7.4, 18.3, 72.9 and 107.3 kJ day higher in the thermophilic than the mesophilic digestor. CH4 production decreased more rapidly with each increase in VS loading rate and decrease in retention time (RT) in the mesophilic than the thermophilic digestor. When expressed as l g(-1) VS fed or as kJ kJ(-1) fed, the amount of CH4 was 49% less at the highest compared to the lowest loading rate in the mesophilic digestor. In the thermophilic digestor the decrease was only 16%. Propionate accumulated in the mesophilic digestor at the two highest loading rates, reaching concentrations of about 50 mM, but were only about 13 mM in the thermophilic digestor. Isobutyrate, isovalerate plus 2-methylbutyrate, and valerate also accumulated at the higher loading rates.