Recent seismological studies of the Coso region of southeastern California document both low P wave velocities and abnormal SV attenuation in Indian Well Valley, south of the Pleistocene volcanic of the Coso Range. In order to learn more about the physical nature of these colocated anomalies, a tomographic inversion for the three-dimensional variations of Vp/Vs, the ratio of compressional to shear velocity was performed. Iterative back projection of 2966 shear and compressional wave travel time residuals from local earthquakes recorded on vertical instruments reveals that Vp/Vs is generally high at the surface and decreases systematically to 10 km depth. Superimposed on this trend are several large anomalies. Near Devil's Kitchen in the Coso Geothermal Area, Vp/Vs values are very low near the surface, consistent with measured values for steam-dominated geothermal systems. Abnormally high values of Vp/Vs are observed in portions of Indian Wells Valley and also below the Cactus Peak rhyolite dome from 2 to 5 km in depth. In Indian Wells Valley the co-occurrence of low P velocities, low S velocities, high Vp/Vs, and anomalous SV attenuation are indicative of subsurface partial melt. Because the inversion is based on vertical data only, however, the results cannot be considered conclusive, only suggestive. ¿ American Geophysical Union 1988