Abstract The Ozark and Lincoln fold areas of Missouri (Fig. 43) include 74 counties comprising approximately 45,400 sq mi (117,590 sq km). The part south of the Missouri River, consisting of 52 counties (33,100 sq mi or 85,730 sq km), covers the Ozark area and is largely unprospective as a hydrocarbon province. Possibilities are limited to the counties adjacent to the Mississippi River where oil might be found in Cambrian rocks, but the potential of these rocks is impossible to evaluate at this time. The 22 counties (12,300 sq mi or 31,860 sq km) north of the Missouri River, including the Lincoln fold area, have a cover of post— Lower Ordovician rocks which makes them more attractive for oil prospecting. However, the absence of the Maquoketa Shale and of the Silurian reefs—present in Illinois on the east and south—limits possibilities for oil accumulation in northeastern Missouri. St. Charles County north of the river and St. Louis County south of the river have similar potentials and constitute a third possible area of interest. At present, the only production is from the Florissant field in St. Louis County; it produces from the Kimmswick Limestone of late Middle Ordovician age. Total production to January 1, 1968, was approximately 700,000 bbl of 35° gravity oil. Future production of petroleum probably will not amount to more than 2-3 million bbl (from the Florissant field and the development of other small fields in the post—St. Peter rocks).