The Yellowstone volcanic complex (YVC) contains the largest concentration of geothermal features in the world with the geothermal features being formed by a large magmatic system that underlies the YVC. This magmatic system is thought to be caused by a stationary hot spot source, however, recent seismic and magnetotelluric studies support both a stationary hot spot and nonhot spot sources. Most of the geothermal features occur within an approximately 30 km wide caldera and gravity studies have been important in determining the crustal structure of the caldera with the latest caldera complex being associated with a regional gravity minimum. Despite all the regional gravity interpretations which show the thickness of the crust and gravity minima associated with partial melting areas, there is a lack of detailed data except in a few geothermal basins that were part of investigations to determine how gravity changes over time. In this study, I have collected over 300 gravity stations spaced between 300 and 400 meters between the Lone Star Geyser region and the Lower Geyser Basin in the western portions of the caldera. This region includes the Upper and Midway Geyser basins and contain the highest concentration of geothermal features in the world. The stations collected using differential GPS and processed using terrain corrections were merged with existing data to produce a complete Bouguer gravity anomaly map. Residual gravity anomaly maps created using wavelength filtering indicates that the Mallard Dome Lake Dome, the Lone Star Geyser region and the northern section of the Lower Geyser Basin are associated with gravity minima probably associated with deep hydrothermal alteration. The entire region from the northern boundary of the Lower Geyser Basin and the southern edge of the Upper Geyser Basin is characterized by a gravity minimum and preliminary modeling suggests that this region is underlain by a thick region of altered rhyolite similar to that exposed in the Yellowstone Grand Canyon. However, there are several exceptions including the Upper Geyser Basin is on the edge of a gravity maximum suggests that the deeper alteration of the rhyolitc rocks is not as intense.