Forty-five earthquakes were detected and six were located during an 11-day reconnaissance of microearthquake activity in southeastern Missouri and western Tennessee. These are some of the smallest magnitude earthquakes ever identified in this region, although seismic activity there has been observed from permanent stations for many years. In this program, four high-gain portable seismographs were used to occupy 12 recording sites in the Ozark Uplift in southeastern Missouri and the New Madrid seismic zone of the Upper Mississippi River Embayment. Although our data are sparse, observed microearthquake activity in the New Madrid seismic zone was found to be much greater than on the Ozark Uplift in agreement with long-term observations in both areas. Earthquakes as shallow as 1 km were confirmed in the New Madrid seismic zone. There, also, a b-value of 0.6 for a range of 6 magnitude units was determined from the microearthquake data of this investigation and Saint Louis University's macroseismic data for the period 1950 to 1964.