Hydrothermal fluid flow can be visualized by three-dimensional (3-D) kriging and computer contouring of oxygen isotope data on propylitized andesites in the Comstock Lode mining district. This data set consists of δ18O results for 327 samples from surface outcrops, drill cores, and mine workings in a 3.5 km ¿ 1.7 km ¿ 1.0 km block in the hanging wall of the Comstock fault encompassing the region where the most important ore bodies were discovered. The δ18O contours deep in the hanging wall are not simply parallel to the Comstock fault or to the boundary of the Davidson granodiorite stock as would be expected for a hydrothermal convection system dominated by unicellular flow. Instead, the subsurface δ18O values exhibit a high degree of symmetry about a vertical plane directed radially toward the stock, with isotopic contours being predominantly perpendicular to the Comstock fault and the eastern contact of the Davidson granodiorite. A vertical cross section along the Comstock Lode shows a low δ18O core with two symmetrical zones of higher δ18O values to either side where the contours are steeply inclined, which we infer reflects downwelling of fluids shifted by water-rock exchange or steam loss. This 3-D visualization of δ18O values provides the first evidence that longitudinal convective rolls can be significant in geologic environments, in this case within a zone of high temperature gradients and permeability near the Comstock fault and the Davidson granodiorite stock.