The kinetics of biological slime films were investigated using six completely mixed, annular reactors. Design criteria for trickling filters were formerly based on experience and guesswork, so this investigation attempted to determine the kinetics of a simple system to provide a better basis for establishing trickling filter design criteria. A simple glucose and mineral base substrate was fed to microorganisms attached to the walls of the 6 annular, completely mixed reactors. The reactors were operated first in parallel, for a basic investigation of biological film kinetics, and second, in series, to approximate plug flow. Completely mixed reactors eliminated channeling, dissolved oxygen and substrate gradients, and the interdependence of velocity and flow. Film development was uniform, and liquid channeling did not occur. This investigation revealed that biological films develop in three distinct stages: (1) logarithmic growth which lasted until the active thickness was reached; (2) linear growth, between the active thickness and the plateau thickness and (3) zero growth, as all new cells are washed away. The active film thickness averaged 70 and was independent of D.O. Kinetic equations were developed which showed good correlation between theoretical and observed data. On this basis, a trickling filter design equation, based on the kinetics of the process, was developed. (WRSIC abstract)