Conodonts are widely distributed both stratigraphically and geographically in rocks of Devonian and Mississippian age in the upper Mississippi Valley. The stratigraphic distribution of conodonts and the relationship of their occurrence to lithology indicate that conodonts were continuously present and rather uniformly abundant in the seas during Devonian-Mississippian time. Accordingly, the rate of deposition of sediments may be the most important factor governing conodont occurrence. In sandstones, such as those of the Chester series, sedimentation rates were high and the sandstones are virtually barren of conodonts. Kinderhook shales, on the other hand, are the result of irregular rates of clay and silt deposition and contain many zones of very high conodont yields interspaced with zones of low or no yield. In most of the limestones, stratigraphic distribution of conodonts is much more uniform than in shale samples, but average yields are lower. Most limestones thus appear to be the result of moderately slow, but rather constant, deposition. A paucity of conodonts in the Louisiana, McCraney, and Glen Park limestones may indicate that those formations are the result of rapid deposition. Bottom environment appears to be a minor factor in the distribution of conodonts in marine rocks. Rexroad has shown by study of the Glen Dean that the nature of the conodont fauna is not significantly affected by lithologic variations. Conodonts apparently do not occur in nonmarine or brackish-water sediments.