The present invention relates to a safety binding of a boot on a ski which comprises two lateral jaws (2) mounted for movement under the force of an elastic member (5) between a closed service position in which these jaws coact with the sole of the boot (C) and an open position. It also comprises structure (3) interacting with the heel of the ski boot and having a sole grip member (29) mounted for movement by the force of the same elastic member between a service position in which this member maintains the rear end of the sole of the boot on the ski and an open position, the whole being so arranged that the opening or the closing of the lateral jaws effects the opening or the closing respectively of the sole grip member, and vice versa.