A three-stage continuous culture model system was used to determine the effects of temperature on methanogenic associations enriched and isolated from landfill sites. This system allowed spatial segregation of the trophic groups of the associations without disruption of the integrity of the associations. Three intermediates of anoxic catabolism in landfill sites were selected as the carbon sources for the study : cellobiose, butyrate and valerate. No evidence of thermophilic methanogenic activity was found in any of the systems. However, some thermophilic sulphate-reducing activity was detected. No inhibition of the cellobiose-degrading bacteria was evident over the range 20-55 degrees C. The optimum temperature range for methanogenesis was generally 30-35 degrees C, although there were apparently two methanogenic populations, one of which maintained methane release rates over the range 20-35 degrees C.