Poseidon regulators have a unique oversized mouthpiece retention lip that no ordinary mouthpiece will fit properly. This mouthpiece properly fits the Xstream, Jetstream and Cyklon 5000 second stages. Also fits the Poseidon MKVI and Se7en rebreather DSV (bailout valve), and the Dive Rite Jetstream RG3500 second stage (a re-branded Poseidon). This mouthpiece is also unique in that it is fully symmetrical in order to support the ambidextrous feature of the Poseidon second stages.

You might also consider the model #5 of the X-Type Moldable Mouthpiece by SeaCure as a more comfortable and perhaps more durable alternative that is designed specifically to fit the Poseidon opening.