AbstractA two‐member methane‐utilizing mixed culture of bacteria, formed by combining two pure cultures isolated from a naturally occurring methane‐utilizing mixed culture, was studied in continuous culture. From the nutritional requirements and substrate ranges of the pure cultures, a mechanism for the interspecific interactions occurring in the mixed culture was proposed. Product formation kinetics were determined in continuous culture for each product involved in the proposed mechanism. From this proposed mechanism a mathematical model was derived based on simple material balance equations around a single‐stage chemostat. The steady‐state predictions of this model were compared to experimental results obtained from continuous‐culture experiments with the two‐member methane‐utilizing mixed culture. Interspecific interactions occurring in two‐member methanol‐utilizing and three‐member methane‐utilizing mixed cultures have also been discussed.