A regional ground-water model of the Floridan aquifer was constructed for an 875-square-mile portion of the rapidly developing area north of Tampa Bay. The digital model was calibrated by comparing observed (March 1974 and May 1975) and computed potentiometric heads. A good comparison was obtained by adjusting leakance and transmissivity. Differences between the computed and measured potentiometric surface were generally less than 3 feet, with a maximum error of 15 feet. The calibrated model may be used as a predictive tool. For example, the model could be used to evaluate the regional effects of increased ground-water withdrawal on the Florida aquifer. (Woodard-USGS)