Primitive meteorites (such as the unequilibrated ordinary chondrites) have undergone only minor thermal processing on their parent asteroids, and thus provide relatively unaltered isotopic records from the early Solar System, For terrestrial materials, oxygen isotope compositions form a linear array called the terrestrial fractionation line(1). In meteorites the oxygen isotopic composition commonly deviates from this line(2), the magnitude of the deviation being expressed by the quantity Delta(17)O. Such deviations, which cannot be explained by mass-dependent fractionation processes, are probably caused by the mixing of two or more nebular components having different nucleosynthetic histories, for example, solids and gas. But no direct evidence for the oxygen isotopic composition of the latter (which is the dominant oxygen reservoir) has hitherto been available. Here we report in situ oxygen-isotope measurements of magnetite grains in unequilibrated ordinary chondrites. Magnetite (which formed by aqueous alteration of metal in the parent asteroid) may serve as a proxy for nebular H2O. We measured a value of Delta(17)O approximate to 5 parts per thousand, much higher than typical values of 0-2 parts per thousand in ordinary-chondrite silicate grains, Our results imply that a nebular component of high-Delta(17)O H2O was incorporated into the parent asteroid of the unequilibrated ordinary chondrites.