A large positive gravity anomaly occurs in an area more than 20 km wide and 80 km long in the eastern Klamath Mountains south of Yreka, California. The maximum amplitude of the residual anomaly is about 50 mgal, the maximum gradient 6.6 mg/km, and the average half-width 8.5 km. Analysis of the anomaly leads to a calculation of 23 × 1010 g/cm for its total amount of anomalous mass per unit length. The maximum possible depth to the anomalous mass is 4 km. Geologic interpretation suggests that the anomaly is caused by a large deformed ultramafic intrusion which is serpentinized at very shallow depths.