The piezoresistive properties ofn‐ andp‐type layers formed by the diffusion of impurities into silicon have been investigated. The values of the three piezoresistance coefficients and the temperature dependence of the large coefficients have been measured on layers having surface concentration values from 1018to 1021cm−3. The piezoresistance effect inp‐type diffused layers follows qualitatively the behavior expected in a degenerate semiconductor.n‐type layers having high surface concentration values show a change in the symmetry of the piezoresistance effect at room temperature and a decrease in the coefficient &pgr;11at lower temperatures. A discussion of the piezoresistance effect in diffused layers and its relation to the piezoresistance effect in uniformly doped material is also given.