Maps of the potentiometric surface of the Floridan aquifer in west-central Florida are prepared semiannually by the U.S. Geological Survey in cooperation with the Southwest Florida Water Management District. Maps for May and September show the potentiometric surface that is close to the annual low or high water level condition that usually occurs in May or September, respectively. The surface is mapped by determining the altitude of water levels in a network of wells and is represented on maps by contours that connect points of equal altitude. This report shows the potentiometric surface of the Floridan aquifer for September 1983. Data for the map were collected during September 12-23. The map represents water level conditions near the end of the summer rainy season when groundwater withdrawals for agricultural use are low. Water levels in most wells measured in September 1983 were higher than those measured in May 1983. September water levels averaged about 0.6 ft higher than May levels in areas north of Hillsborough County and about 7 ft higher in southern areas. Water level increases in the north ranged from zero to about 2 ft along coastal regions and from zero to about 5 ft in inland areas. Water level declines in the north ranged from zero to about 7 ft and occurred in scattered parts of all northern counties. In the south, water level increases ranged from zero to about 3 ft along coastal and extreme southern regions and from about 5 to 34 ft in other southern areas. The greatest water level increases occurred in southern Hillsborough County where irrigation pumpage contributes to large seasonal fluctuations. There were a few areas in Hillsborough and Pinellas Counties where September water levels showed declines ranging from about 1 to 3 ft when compared with May levels. (Lantz-PTT)