Abstract: A tractor includes a body frame (4), front wheels (3), propelling shafts (8) disposed in a rear portion of the body frame, and semicrawler units (10) for receiving drive from the propelling drive shafts. Each semicrawler unit includes a drive rolling element (11) attachable to one of the propelling drive shafts, an endless crawler track (15) driven by the drive rolling element and contacting the ground, driven rolling elements (13, 14) engaged by the endless crawler track, and a track frame (12) for supporting the driven rolling elements. The track frame is supported by the body frame to be vertically oscillatable through a pivot shaft (16). Rear wheels (9) may be attached in place of the semicrawler units to the propelling drive shafts.