Carbon isotopes in fossil emu (Dromaius novaehollandiae) eggshell from Lake Lyre, South Australia,demonstrate that the relative abundance of C-4 grasses varied substantially during the past 65,000 years. Currently, C-4 grasses are more abundant in regions that are increasingly affected by warm-season precipitation. Thus, an expansion of C-4 grasses Likely reflects an increase in the relative effectiveness of the Australian summer monsoon, which controls summer precipitation over Lake Lyre. The data imply that the Australian monsoon was most effective between 45,000 and 65,000 years ago, Least effective during the Last Glacial Maximum, and moderately effective during the Holocene.