Observations of relative P-wave travel-time residuals for local mantle earthquakes and P/S amplitude ratios for more distant events have been used to investigate the nature of the crust under the hydrothermal areas of the central Taupo Volcanic Zone. In contrast to some other regions of similar activity, such as the Yellowstone and Geysers geothermal areas, U.S.A., there is no evidence in the data for an extensive region of semi-molten or molten rock in the crust. Thus the large amounts of mafic intrusive rock and associated crustal melt inferred to be present on geological grounds must be now largely at temperatures below the appropriate solidus. The data do provide further evidence for previously suspected lateral inhomogeneity in the underlying mantle.