An algorithm for automatically guiding anaerobic digesters to their optimum temperature is presented. The algorithm performs ''adaptive optimization'', i.e. relies on a model that is continually adapted to reflect the current state of the digester being optimized. It will lead a digester to its optimum steady-state temperature whether this is in the mesophilic or in the thermophilic range. Experimental results of its application to a continuous glucose-fed anaerobic digester are presented.