A full-scale aerobic digestion system demonstrated that a simple self-aspirating aerator, that used ambient air, could achieve high oxygen transfer efficiencies and thereby allow conservation of heat. Continuous feed operation utilizing primary and waste activated sludges resulted in autoheated reactor temperatures ranging from 45 to 65C, even when ambient air temperatures were -20C and sludge temperatures were 0C. Process variables included organic loading rate and D.O. concentration. It was observed that intermediate loading rates and low D.O. residuals (less than 1 ppm) allowed maximum temperature development. The potential of the autoheated thermophilic digester to inactivate pathogens was investigated. Virus inactivation was 100%. Bacterial and parasite counts were less than those found in mesophilic anaerobic digestion sludge. Dewaterability of the aerobically treated sludge deteriorated at all loading conditions studied. This last feature was the only disadvantage evident in the process.