Six carbonate sections of the upper half of the Chester Series (Glen Dean Formation to Kinkaid Formation) in type area of Southwestern Illinois were sampled at an average interval of 15 centimeters. Nearly four hundred thin sections were analyzed in a statistical petrographic (microfacies) investigation. Frequency indices for crinoid and bryozoan debris, ostracods, calcified monaxonic sponige spicules, arenaceous foraminifers, calcispheres, detrital quartz and oolites were measured. Indices of clasticity were also computed for crinoid debris, detrital quartz and oolites. Curves of the stratigraphic variation of each parameter were plotted and used for the interpretation of the environment of deposition. The statistical data combined with textural properties and degree of recrystallization revealed an association, at times regularly cyclical, of five distinct and environmentally control1ed microfacies. These are in order of increasing energy level or decreasing relative depth: calcisiltite with scattered debris, mud-supported biocalcarenite and three types of grain-supported biocalcarenites with calcisiltite matrix recrystallized to a variable degree. All these microfacies indicate an environment of general low energy in which moderate winnowing processes were active. In the shallower portions where crinoid and bryozoan colonies thrived, they led to the inflow of transported oolites, the sorting of crinoid debris and the fragmentation of bryozoan fronds almost in place, In the deeper portions of the environment where abundant siliceous sponges and scavengers were associated with small crinoid clumps, the winnowing processes concentrated the silt-size detrital quartz and all the small organic components such as arenaceous foraminifers, ostracods and calcispheres originally distributed throughout the environment.