Oxygen is the most abundant element in the solar system after H and He, and the lightest element with three stable isotopes. Oxygen is a volatile element; it was not retained completely in meteorites or in their predecessors during their formation in the early solar system. The solar oxygen isotopic composition is important because the Sun is by far the largest reservoir of oxygen in the solar system. Oxygen occurs in highly volatile phases of the solar system, such as water, as well as in refractory phases, for example in silicates. Because of this, the differences between the isotopic composition of oxygen in solar-system bodies and the Sun are a sensitive measure of the gas to dust ratio in the early solar system. We report the first measurements of the oxygen isotopic composition in the fast solar wind with the Solar Wind Isotope Mass Spectrometer (SWIMS) on the Advanced Composition Explorer (ACE). The fast solar wind is the least fractionated type of solar wind. We obtain an isotopic abundance ratio 16O/18O of 446¿90 which is (within the quoted uncertainties) consistent with the terrestrial value of 498. However, in combination with previously published values for the photosphere (16O/18O=440¿50 [Harris et al., 1987>) and the isotopically more fractionated slow solar wind (16O/18O=450¿130 [Collier et al., 1998>), this new determination may suggest that the Sun is isotopically heavier than terrestrial. ¿ 2001 American Geophysical Union