AbstractThe bacteriumAcetobacter suboyxdansand the yeastSaccharomyces carlsbergensishave been grown together on a synthetic medium in a chemostat. Mannitol, the only carbon source fed to the fermenter, is oxidized by the bacteria to fructose. The yeast, which cannot attack mannitol, breaks down the fructose nearly completely. Eight steady states and five transitory periods after changes in flow rate have been analyzed to study the kinetics of the mixed culture. Separate cell concentrations were determined by a modified Coulter counter apparatus. Both sugars were monitored. Both bacteria and yeast may be modeled using Monod's equation, the latter with some deviations. The yeast is unable to grow beyond the washout point of the bacteria, even though its maximum growth rate is much higher. The yield of both organisms decreases with increasing dilution rate, as does their average cell size. After step changes in dilution rate, repeated oscillations of both sugar and cell concentrations usually occur before steady‐state conditions are reattained. They are generally in phase, with no definite sign of a lag. Oscillations of yeast and fructose concentrations are more pronounced. Periods average about 6 hr and are not correlated with fermentation conditions or equipment variables. Repeated oscillations are not found after step‐downs in pure cultures ofA. suboxydans, leading to the conclusion that the instability in mixed cultures may be caused by a feedback mechanism from the yeast to the bacte