An automatic valve for a sewer or the like includes a body having an upstream inlet and a downstream exit for liquid. A path joins the inlet and exit, and a closure gate is hingedly mounted to the body at a substantially horizontal axis located above the path, so that the gate can swing between a first position in which it blocks the path and a second position in which it unblocks the path. An arm is fixed with respect to the closure gate to swing with it in such a way that the arm is higher when the gate blocks the path than it is when the gate unblocks the path, the arm counterbalancing the closure gate so that the latter normally seeks the second position. A cage surrounds the arm and communicates with the path, and a float member in the cage under the arm means is free of connection to the arm means or any other part of the apparatus, whereby a rise in liquid level causes the float member to urge upwardly against the arm and close the closure gate.