The Mount Eden fauna, first described by Childs Frick in 1921, is an important late Hemphillian mammalian fauna from the San Timoteo Badlands of southern California. Identification ofRepomys maxumiMay, an early hypsodont cricetid rodent, and the results of geomagnetic-polarity determinations indicate that this fauna correlates with the lower reversed part of the Gilbert Magnetic Polarity Chron and must be older than the C2 (Thvera) normal-polarity event of this Chron. The fauna is between 5.0 and 5.4 m.y. old according to the temporal calibration of the geomagnetic stratigraphy here followed. This age approximately dates the change from Peninsular Ranges source to a Transverse Ranges source for sediment in the San Timoteo Badlands.