The trap is designed as a floatable tube (1) having at least one end an inwardly pointing funnel (2,3) which is connected gas-tight to the pipe wall and has an opening (4,5) at the point. The internal width of the opening matches the size of the rats. A transparent flap (6,7) is attached to the funnel so that it closes the opening but can move inwards to the inside of the tube. Gas to kill the rats is provided in the tube but only filling out a part of the space enclosed by same. The tube can be attached to a gas supply for automatically topping up the amount of gas. A float (8) can be fitted to the outside of the tube.
