A total of 94 teleseismic events of good quality were recorded by a dense mobile array of seismographs located in the Mono Craters volcanic area, eastern California, one of the youngest apparently active rhyolitic volcanic centers in North America. An inversion of travel time residuals from these events reveals a small anomalous volume, 200--600 km3, directly beneath the Mono Craters with at least 7% low velocity and a top approximately 8--10 km deep. It reasonably may be interpreted as a magma chamber of molten or partially molten rock, although smaller, shallower, and differently placed than previously thought. The magma chamber probably is too small and young to produce a caldera-forming eruption within the foreseeable future.