Surface‐potential decay in naphthalene single crystals was measured after a negative corona discharge on the samples. These potential decay curves do not reach zero value, which is attributed to charges which have been trapped on their way through the sample. The value found for the trap‐modulated mobility at room temperature was 10−8cm2 V−1 s−1and its dependence on temperature was adequately described by an Arrhenius equation, with activation energy of (0.60±0.03) eV.