A gaming machine includes a display on which a plurality of display of display positions are displayed. A game controller is arranged to control images displayed on the display. The controller also controls playing of a game on the machine wherein a plurality of symbols are randomly selected and displayed on the display such that if a winning combination results, the machine pays a prize. The controller is operable, upon the occurrence of a predetermined triggering event, to generate a bonus symbol which is displayed in a first display position and, thereafter in at least one other display position. In predetermined circumstances, when the bonus symbol moves from a preceding display position, a residual symbol remains at the preceding display position and, when the bonus symbol appears at a subsequent display position, both the original bonus symbol and the residual bonus symbol operates as substitute symbols in their display positions.