Oaks and their associated gallwasps are often cited as a classic example of coevolution in a plant-insect system. Therefore, it is expected that these gallwasps should demonstrate a high degree of host fidelity over their evolutionary history. To test this, we studied 25 fossil floras from the Oligocene through Pliocene of the western United States. Galls were found on the leaves of Quercus simulata-type Knowlton, 1898 and Q. pollardiana-type (Knowlton) Axelrod, 1940, and match previously reported galls of the form genus Antronoides Waggoner and Poteet, 1996. These fossil leaf-galls are similar to those made on oaks by modern Cynipini wasps, and were restricted to two oak species. Galls are present on these oaks in western floras for a span of 30 million years, indicative of remarkable host fidelity through time. The distribution of galled leaves from across the western United States indicates that the radiation of this group of gallwasps occurred more or less simultaneously across a wide geographic area.