Abstract: A mouthpiece regulator for an underwater breathing apparatus utilizes a unitary valve member to perform both inhalation pressure sensing and inlet valve functions. The valve member includes a hollow stem that projects into an inlet chamber in the regulator housing. The stem end cooperates with a valve seat to function as an inlet valve. The valve member also has a wide flange at the other end of the stem. The flange divides the regulator interior chamber into two regions, one communicating with the mouthpiece, the other communicating via ports with the ambient environment. During inhalation the environmental air or water pressure exerted on the flange causes the inlet valve to open, supplying air to the mouthpiece via the hollow stem. At the end of inhalation, increased mouthpiece pressure and a bias spring together cause the unitary valve member to close the inlet valve. Exhaled gases are expelled via a flap valve covering exhaust openings through the flange.