Ground water from the major Jurassic aquifer in the Great Artesian Basin, Australia, shows significant variation in calcium and strontium concentration and strontium isotopic composition with age and distance from the recharge area. Hydrologically young bore waters in the eastern part of the basin exhibit surprisingly unradiogenic 87Sr/86Sr ratios (0.7045 to 0.7054). By contrast, hydrologically older waters in the central and discharge regions of the basin have significantly more radiogenic isotopic compositions (0.7060 to 0.7118). Waters with unradiogenic 87Sr/86Sr ratios are interpreted to be the result of reaction between recharge water in the aquifer and lithologies that have juvenile 87Sr/86Sr compositions. These juvenile strontium isotope signatures most likely reflect interaction with Cenozoic mafic igneous rocks which occur on the eastern side of the basin. More radiogenic waters in the basin are the result of addition of 87Sr derived by in situ dissolution of aquifer silicates.