The effect of energetic electrons on the evolution of a tearing mode (m/n = 2/1, where m and n are the poloidal and toroidal mode number, respectively) has been identified in the Rutherford (i.e. magnetic island) regime for the first time in the HT-7 tokamak, where energetic electrons were generated by the residual loop voltage and the varying background electron density in the ohmic plasma. The tearing mode rotated in the electron diamagnetic drift direction and developed two distinct types of behaviour. One has a lower background electron density characterized by the frequency chirping down where the magnetic island grows linearly to a critical width and is then periodically stabilized. The other has a higher electron density with the frequency chirping down and jumping up where the magnetic island never surpasses the critical width. Comparisons with calculations show that the evolution is influenced by energetic electrons in different collisional regimes.