The extent of lateral inhomogeneity in the uppermost mantle beneath New Zealand has been studied by mapping the variation of Pn and Sn velocities. Ten regions of constant structure were required to model the distribution of velocities, which range from 7·4 km/s and 3· 95 km/s for Pn and Sn respectively, beneath the central North Island, to 8· 7 km/s and 4· 8 krn/s beneath Nelson. It is suggested that the regions partition the uppermost mantle according to whether its constituents are ultramafic or mafic and according to the concentrations of open cracks and melt.