Fastening device for an infant seat

The fastener part (6) of the fastener device (5) can be moved between a working position and an idle position in the seat length direction (8), ie at right angles to the slit (4) between the seat cushion (3) and backrest (2). The fastener device comprises at least one mounting which is fixed to the vehicle seat (1) and acts as a holder for a fastener part that can be moved between a working position in which it extends through the slit between the seat cushion and backrest, and an idle position in which it is lowered into this slit. An Independent claim is also included for a vehicle seat fitted with this fastener device.