Two microorganisms, E. coli and S. cerevisiae, competing for glucose were maintained in a stable cycle of COexistence by alternating the growth advantage between the two organisms by oscillating the pH in a chemostat. Pure culture experiments found S. cerevisiae to be insensitive to pH between 5 and 4.3 with a maximum specific growth rate (p,J of 0.4/hr; while pmax of E. coli decreased from 0.6 h -' at pH 5 to 0.1 h -' at pH 4.3. Steady-state and cross-inoculation chemostat runs at a dilution rate of 0.17 h-' confirmed the expectation that the mixed culture system is unstable at constant pH with E. coli dominating at pH 5 and S. cerevisiae dominating at pH 4.3. Three pH oscillation experiments were performed at D = 0.17 h-' with 1 g per liter glucose feed. The 16 hi16 h cycle was stable for six periods with a stable alternating cycle of E. coli and S. cerevisiae being quickly established. A 18 h pH 5/14 h pH 4.3 cycle was found to be stable with smaller yeast concentrations. A 6 h/6 h cycle was found unstable with yeast washout. Simulation results were compared with these runs and were used to predict the onset of instability. Oscillations of pH can force stable persistence of a competing mixed culture that is otherwise unstable. Thus, varying conditions are experimentally demonstrated to be one explanation for competitive coexistence.