Oxygen stable isotopes were measured in whole sagittae of young Pacific sardine (δ18Ooto) collected throughout their range in the Mexican Pacific to quantify natural variability, to reconstruct temperature histories, and to infer whether fish mix at a population or subpopulation level. Isotopic values and derived temperature estimates (T oto) of sardine captured simultaneously showed high variability (up to 2.0‰ and 10°C at a given location). Given limited variations in salinity, this implies differences in thermal history and the prevalence of subpopulation-level mixing processes. We tested the null hypothesis of local residence by comparing δ18Ooto values with predicted isotopic values on a location-specific basis, and age- and location-specific average sea surface temperatures (SSTs) with derived Toto. Some fish exhibited values outside the local range of predicted oxygen isotope values and SSTs, suggesting that they were not permanent residents. Using an otolith growth model, we show that otolith growth and age differences cannot fully account for the variability in δ18Ooto values. The absence of significant differences in δ18Ooto values between the Pacific and Gulf indicates that oxygen isotope ratios cannot be used to examine population structure or migration among these regions. However, they can be used to infer mixing within and among subpopulations. © 2008 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved.