A duckbill valve is provided which includes a hollow cylindrical elastomeric body with first and second ends. The first end has walls parabolically tapering to an elongate slit, the slit being surrounded by an upper and lower lip. The second end has a non-uniformly round (e.g. oval) outer wall terminating in an open mouth. When inserted into a perfectly round apertured fitting, the non-uniformly round outer walled elastomeric body is bowed to form-fit the round configuration thereby forcing shut the lips and slit. Upon increased air pressure within the elastomeric body, the slit is forced open. In a second embodiment, the inner wall of the second end may be non-uniformly (e.g. oval) round. Insertion of a round tubular fitting within the non-round inner wall of the elastomeric body also achieves a bowed form-fit configuration which forces shut the lips and slit.