It is shown that the distribution function assumed by Mott‐Smith determines a unique relation between heat flux, stress, and fluid velocity given byq = (3/2)&tgr;u, i.e., it provides a constitutive relation for heat flux, and it also determines a simple expression for the ratio of third‐order central momentsQ = ⟨C x3 ⟩ / ⟨C x C 2⟩. These expressions allow the equation of transfer forcx2to be cast in a form that yields a nonlinear constitutive relation for stress. The results obtained from the Mott‐Smith ansatz are compared with the theory of Baganoff and Nathenson and results from a numerical solution of the Boltzmann equation for shock‐wave structure obtained by Hicks and Yen.