The stable carbon and oxygen isotope compositions of serial samples of enamel from tusks of Gomphotherium productus (Mammalia, Proboscidea) from Port of Entry Pit, Oklahoma (early Hemphillian North American Land Mammal Age, ca. 7.5 Ma), were measured to examine intra-annual and interannual variation. Sample series from each of six tusks spanned approximately one year of tusk growth. Carbon isotope compositions range from −11.3 to −9.2‰ (VPDB) and exhibit no pattern of seasonal variation, indicating the diet of gomphotheres at Port of Entry Pit was dominated by C3 vegetation throughout the year. Phosphate oxygen isotope composition (δ18Op) ranges from 18.9 to 22.2‰ (VSMOW); carbonate oxygen isotope composition (δ18Oc) ranges from 26.1 to 30.1‰ (VSMOW). None of the tusks exhibit seasonal variation in δ18O, and the average within-tusk range in δ18Op is 1.7‰. Neither the fluorine composition of the specimens nor the relationship between δ18Op and δ18Oc values from splits of the same samples suggest significant post-depositional alteration. The oxygen isotope data imply that Hemphillian meteoric water had δ18O values that are indistinguishable from modern values in the region today. However, because polar ice sheets were smaller and mean ocean water δ18O lower than present, the estimates of meteoric water composition from gomphothere tusk δ18Op are consistent with warmer mean annual temperatures during the Hemphillian. The within-tusk variations in δ18Op are consistent with similar or much reduced patterns of seasonal temperature variation in comparison to today, depending on air mass flow during the late Miocene. Alternative explanations include seasonal migratory behavior and reliance by the gomphotheres at Port of Entry Pit on sources of drinking water with relatively constant δ18O values.