Thermophilic sulfate‐reducing and methanogenic bacteria were detected in deep (1200 to 1800 m) ground waters from geologic units that make up the Madison Formation, which underlies a large portion of the northern Great Plains. Some sulfate‐reducing bacteria were isolated and tentatively identified asDesulfotomaculum nigrificans.These organisms probably produce the hydrogen sulfide that is in the ground water. Direct microscopic counting methods demonstrated the presence of about 1000 bacteria/ml in the ground waters. Attempts to detect aerobic and other anaerobic bacteria were unsuccessful.