Radiocarbon variability in Porites spp. corals from Guam and the Makassar Strait (Indonesian Seaway) was used to identify the source waters contributing to the Indonesian throughflow. Time series with bimonthly resolution were constructed using accelerator mass spectrometry. The seasonal variability ranges from 15 to 60?, with large interannual variability. Δ14C values from Indonesia and Guam have a nearly identical range. Annual mean Δ14C values from Indonesia are 50 to 60? higher than in corals from Canton in the South Equatorial Current <Druffel, 1987>. These observations support a year-round North Pacific source for the Indonesian throughflow and imply negligible contribution by South Equatorial Current water. The large seasonality in Δ14C values from both sites emphasizes the dynamic behavior of radiocarbon in the surface ocean and suggests that Δ14C time series of similar resolution can help constrain seasonal and interannual changes in ocean circulation in the Pacific over the last several decades.¿ 1997 American Geophysical Union