An adjustable seat is mounted on a base for adjustment of its position longitudinally of a motor vehicle, by means of two guiding arrangements, each located at one side of the seat and each including a front guide track and a rear guide track. The seat is arrestable in a multitude of adjusted positions by at least one arresting mechanism which includes a rack mounted on the base, a pinion mounted on the seat for rotation in mesh with the rack, a ratchet connected to the pinion for joint rotation therewith, and a pawl displaceably mounted on the seat and engaging the ratchet to arrest the seat in any selected one of the multitude of adjusted positions. The rack may extend parallel to one of the guide tracks and then a support member which supports the seat on this guide track may be mounted coaxially with the pinion. Another such arresting mechanism may be arranged at the opposite side of the seat from the above arresting mechanism, and a shaft may interconnect the pawl of the two arresting mechanism for simultaneous displacement.