Granulite-facies ortho- and paragneisses from the Mallee Bore area, central Australia are depleted in 18O by up to 10%o. The zone of 18O depletion is elongate, not coincident with any lithological or structural feature, and covers an area of approximately 9 km x 3 km. Within this zone, average 818O values of paragneisses and orthogneisses are c. 4.6%c and c. 4.1%c respectively, and locally are as low as 2.7%o and 2.4%o, respectively. The lowering of δ18O values probably occurred due to isotopic exchange between the rocks and a surficial fluid at high temperatures. The preservation of high-temperature mineral oxygen isotope fractionations and the lack of gross retrogression of the Mallee Bore rocks, indicates that fluid flow after regional metamorphism was negligible. Furthermore, the difficulties associated with downward circulation of surface-derived fluids to depths in excess of 25 km, as well as the lack of wholesale melting in the rocks, suggest that fluid flow occurred prior to the peak of granulite-facies regional metamorphism. Hence, the 18O depletion observed at Mallee Bore most likely occurred during early contact metamorphism, or during the prograde stages of regional metamorphism. The preservation of low δ 18O values suggests that there was little pervasive fluid flow during or subsequent to regional metamorphism.