Abstract: An automatic guidance system for a lawn mower comprises four tension cables (10 to 13) each anchorged at a point (15) adjacent one of the corners of a quadrilateral area and each wound round a winch drum (16 to 19) on the mower, a motor (60) on the mower is arranged to drive the four winch drums selectively in pairs, one drum winding-in while the other winds out, by means of a common gear ring 63 which is shifted into one of four different driving positions determined by a guide plate 68 in a square opening (70). The plate is shifted automatically from one position to the next when the mower approaches any one of the automatically adjustable cleats (20) on the individual cables. The mower thus follows a rectangular path of diminishing size.