Gordonia amarae and Acinetobacter calcoaceticus pure culture experiments were conducted in continuous flow chemostats fed with an acetate/mineral salts medium under aerobic and anaerobic-aerobic conditions. The growth parameters such as volatile suspended solids (VSS) suggest that Acinetobacter can successfully out-compete Gordonia under both aerobic and anaerobic-aerobic conditions when both are present in a dual culture. Gordonia was reduced to near detection limits (<10 mg VSS/l), while Acinetobacter successfully grew to about 300 mg VSS/l when both were grown as a dual culture in a single-stage aerobic chemostat with an initial concentration of 200 mg VSS/l each. In a two-stage anaerobic-aerobic chemostat with recycling, Gordonia was completely washed out in about 24 h, whereas, Acinetobacter successfully grew to over 170% of its initial concentration. In a single-stage aerobic chemostat, a mathematical model describing the competitive growth of the two species based on single culture kinetic parameters predicted Gordonia washout in <30, and the same was observed from experiments. The model predicted an Acinetobacter concentration of 200% of its initial concentration in about 30 h, while experimentally this value was about 150%. Under anaerobic-aerobic conditions, the two-stage model predicted Gordonia washout and an Acinetobacter concentration of about 170% of the initial concentration. The experimental results agreed with the prediction, but the decrease in the Gordonia and increase in the Acinetobacter concentrations were much more rapid than predicted by the model.