Results from a wide range of systems for the continuous fermentation of brewer's wort demonstrate that two criteria determine the operational aspects of such systems. These are: (1) The degree of mixing, and (2) The degree of restraint over the escape of yeast. A reduction in the ability of yeast to ferment occurs when high concentrations of yeast are maintained and is evident to a greater extent in homogeneous systems than in those which maintain a fermentation gradient. Results suggest that satisfactory systems must allow partial escape of yeast and that a minimum degree of heterogeneity should be maintained. These requirements necessitate a system which either maintains a concentration gradient in an elongated vessel or comprises a series of homogeneous vessels with a feed-back of cells so that, in each instance, yeast in the system is regularly exposed to a high concentration of nutrients.