Microfossils have been found to be one of the useful factors in making correlation studies of the Chester formation. Two species of Foraminifera and 32 species and two varieties of Ostracoda constitute the authors' contribution to the microspecimens of the Glen Dean limestone, occurring in the upper part of the middle Chester of southern Illinois. The Ostracoda are classified in 10 families and 22 genera. Four of the genera, 29 of the species, and the two varieties are described as new. The two species of Foraminifera are new forms of the simple coiled types.