A flexible fiber optic bronchoscope with one-way valve is provided in which a unidirectional flow control valve is inserted between the bronchoscopic manifold port and the sealing cap. A compressible diaphragmatic seal with radial slits separate resiliently returnable valve flaps that deformably compress around the shaft of any inserted conduit. A circumscribing housing for typically affixing along a lower receiving ring to the bronchoscopic manifold port to which the sealing cap would normally be attached. An upwardly extending, annular attachment protuberance thereby provides the attachment for the conventional sealing cap.