Compressional and shear wave velocities have been measured in a single crystal kaersutite from an alkali basalt. The kaersutite is highly anisotropic (24% for compressional velocity and 22% for shear velocity) and has a rather low value of Poisson's ratio (0.17). Comparison with velocities measured in hornblendes indicates that kaersutite may have unusually high shear velocities. If kaersutite is present in significant amounts in the upper mantle, its large anisotropy may be important in modeling the seismic structure.