A new, general definition is given for the critical angle of reflection in anisotropic materials. While the critical angle for reflection at a liquid‐isotropic‐solid interface can be written in the formsinθc = vI/vR, we show here that, in general, for an anisotropic solid, the right‐hand side of this expression must be modified by the factor cosφ/cosβ, where φ is the angle between the normal to the refracted wave in the solid and its energy flux vector and β is the angle between the energy flux vector and the component of the refracted wave normal on the interface. Numerical calculations have been made for this general definition for a water‐quartz interface for planes of incidence about thezaxis (thexyDiane being the interface). These calculations are compared with the value of the critical angle as determined by the unmodified definition. It is shown that this earlier definition can lead to large errors when one uses the experimental technique of measuring the critical angle to evaluate the elastic constants of the solid.