Field‐effect mobility measurements on siliconn‐ andp‐channel MOS transistors at temperatures of 4.2 K and lower are reported. The measured field‐effect mobility as a function of the gate voltage shows an anomalous peaked structure superimposed on the well‐known over‐all behavior. These peaks are possibly due to statistical fluctuations within the bands of the semiconductor of the density of surface states as a function of the energy.