Mixed bacterial cultures derived from the rumen were grown in a remen fluid medium in a chemostat at three dilution rates (.02, .06, and .12 per h), each at four growth-limiting glucose concentrations (5.8, 9.9, 12.7, and 25.0 mM). Microscopic observations indicated that a relatively complex mixture of bacterial species was maintained and proportions of fermentations products were similar to those of the rumen except for elevated proportions of methane and acetate. Cell concentration increased linearly with increases in glucose concentration. The range of glucose concentrations had little effect on yields of cells or products produced per mole of glucose fermented. With increases in dilution rates, the amount of butyrate and methane produced per mole of glucose fermented decreased and the amount of propionate increased. Yield glucose (grams cells produced per mole of glucose fermented) increased from 42 at a dilution rate of .02 to 84 at a dilution rate of .12. These large increases are discussed in relationship to the energy requirements for maintenance of bacteria. A theoretical maximum yield glucose of 89.3 and a maintenance requirement of .26 mmol glucose per g cells per h were calculated. Moles of adenosine triphosphate produced per mole of glucose fermented and yield of cells produced per mole of adenosine triphosphate are discussed.