A pontoon crawler track assembly intended to be used as a crawler-track-driven undercarriage in a working machine. The assembly includes a mounting frame and pontoon members. The mounting frame includes a coupler to couple the pontoon crawler track assembly with the working machine and a fastener for attachment of box-structured, hollow pontoon members at the sides of the mounting frame. Each pontoon member includes a crawler track arrangement, which is arranged moveable by way of an internal power transmission arrangement. The breadth of the pontoon crawler track assembly is adjustable. Actuators adjust a breadth of the assembly in a way that a working machine equipped with the pontoon crawler track assembly may be brought, by changing the distance between its pontoon members with the actuators, operating by auxiliary power, first into a narrowed position particularly and on the other hand into a broadened position.