Methods and apparatuses are disclosed for a self-stabilizing ballonet. A bladder or ballonet is provided within an envelope of a balloon. The bladder includes a spheroid body comprising an equator, a top pole, a bottom pole, and an axis crossing through the top pole and the bottom pole. The distance between two poles may be less than the equatorial distance. The bladder is sized and shaped for insertion into an envelope of a balloon and is inflatable to provide altitude control for the balloon.