Experiments are described which establish the existence of a new ‘anelastic piezoresistance’ effect. The effect consists of a change in the electrical resistivity of a sample as a result of stress-induced directional ordering. This change has been detected in polycrystalline Ta–O, Ag–Zn, Ag–In, Cu–Zn and Cu–Mn alloys, which undergo stress-induced ordering by either the Snoek or Zener mechanism. Values obtained for the anelastic strain coefficient of resistivity were, with the exception of the Ag–Zn alloys, much larger than the corresponding elastic strain coefficient. This result suggests that in general the long-range strain surrounding a relaxation centre is not an important factor in determining the resistivity contributed by the centre. It is shown from formal considerations that, for the case of the Snoek relaxation, the anelastic strain coefficient of resistivity is an isotropic crystal property. With this result, data obtained on Ta–O have been analysed to yield the two resistivity parameters which define the anisotropy of the scattering cross section of the interstitial oxygen atoms. For current flow parallel and perpendicular to the tetragonal axis associated with the interstitial oxygen atoms, it is deduced that the resistivity is, respectively, 5·7 and 4·8 μohm cm/at. % oxygen. © 1964 Taylor & Francis Group, LLC.