K-Ar and paleomagntic data from cores through a sequence of basalt flows in the eastern Snake River Plain provide evidence for a brief (0.005 to 0.01 m.y.) reversal of the geomagnetic field 0.46+0.05 m. y. ago. This reversed polarity event has also been found in sea-floor magnetic anomalies and in sediment cores and is probably the Emperor event of Ryan [1972>.