Rorabaugh's (1963) theoretical model of groundwater flow to a stream is used to estimate regional aquifer parameters in the Papaloapan River basin, in southern Mexico. This is a large tropical basin, with 46,517 km² , a diversity of climatic conditions and a substantial interaction between surface and groundwater. Baseflow recession data is used to calculate time of storage, basin constant, and hydraulic diffusivity for ten locations throughout the basin. The results show reasonably good agreement with local geology, as assessed by geologic maps and pumping tests.