Numerous geological and seismological studies of the Long Valley caldera, eastern California, suggest that it is at present an active magmatic system. Tomographic measurements utilising local and regional earthquake data have traditionally been made on the principle 'direct' arrivals. This paper presents the results from a technique developed to image the S-wave attenuation structure. We measure the attenuation value of the S-wave coda from almost 17,000 vertical-component seismograms. A more stable estimate of Q is determined from coda waves which are less sensitive to local path effects than direct waves as they sample a volume. We find that the resultant tomograms produced from spatial stacking show a high attenuation anomaly (Qs~125) centred within the caldera compared with Qs~220 outside the caldera.¿ 1997 American Geophysical Union