A longitudinal adjustment mechanism is provided for a seat, with a spindle guide nut (12) that can move on a spindle (10), which can be driven by a worm gear (14) with bearing sleeves (15) on both sides, as well as with a multi-part casing (20, 30) that receives the worm gear (14) and the spindle (10). The casing (20, 30) consists of two casing sections (20, 30) made of plastic, which can be inserted into one another in the axial direction (X) of the spindle (10); each casing section (20, 30) has a plate (21, 31), from which bars (22, 23, 24; 33) jut out on one side and the casing sections (20, 30) are permanently joined to one another. The mechanism provides a compact transmission for longitudinal seat adjustment mechanisms that is easy to assemble.