A self-sealing balloon or bladder is fabricated of a flexible elastic material and defines a body to which a filler neck is secured. The filler neck is joined to the body to form a fill opening therebetween. The self-sealing structure is provided by bending the filler neck upon and against the surface of the balloon or bladder body. An attachment such as adhesive, tape or sonic welding is utilized to secure the filler neck against the inflatable body. The balloon or bladder may then be filled through the filler neck by the introduction of a flowable material such as a liquid, gas, or powder under pressure to fill and expand the body. Once the filling apparatus has been withdrawn, the elasticity and fill material within the body cooperate to maintain closure of the fill opening.