In 1989 a mark–recapture study of a free-ranging urban population of Pantherophis obsoletus inhabiting the 38.5-ha grounds of the Dallas Zoo (Dallas, Texas) was undertaken and has continued until the present time. Seasonal activity occurred from late February through December. To understand and evaluate further the geographic variation of the seasonal activity of the eastern rat snake complex, we combined data on the seasonal activity of P. obsoletus from our long-term study with those from five other populations previously studied in Texas, Illinois, Kansas, and Maryland and Ontario, Canada. There was a highly significant linear reduction in length of the activity season with increasing latitude. We also describe significant changes in population size and body condition of the snakes during a 26-year study period at the Dallas Zoo and discuss possible factors causing the changes.