Fractionation of oxygen isotopes during crystal growth is commonly assumed to be an equilibrium process. We present evidence here that large variations in δ18O values in a zoned vein quartz crystal are the result of disequilibrium partitioning between fluid and crystal. Within a single crystal with well-developed concentric and sector zoning, δ18O values determined with laser extraction techniques on milligram- to submilligram-sized samples range from 4.7‰ to 7.3‰. Core to rim variations within single growth sectors fluctuate by 2.6‰, whereas differences between adjacent sectors along the same growth zone can exceed 2.0‰. In contrast, fluid-inclusion homogenization temperatures and salinities are relatively constant and show no effect of growth or sector zoning. Differences in δ18O values between sectors along the same growth zone cannot result from variations in the oxygen isotope composition of the fluid and strongly suggest disequilibrium partitioning of oxygen isotopes between fluid and the growing crystal. The cause of this disequilibrium is believed to be due to differences in surface structure and/or growth mechanisms between nonequivalent faces.