Methods and apparatus are disclosed for an apex fitting for securing to a high altitude balloon. An example apparatus involves: (a) a base plate defining an opening, where the base plate is configured to be securable to an exterior of a balloon envelope, (b) at least one stud coupled to the base plate and configured to be securable to a tendon, (c) a retention ring defining at least one opening configured to receive the at least one stud, (d) a fill-port body defining a cavity, wherein a flange is coupled to the fill-port body, wherein the fill-port body is arranged coaxially with and extends through the opening of the base plate such that the flange lies adjacent to the bottom surface of the base plate, and (e) a locking body coupled to the fill-port body, wherein the locking body defines an opening arranged coaxially with the fill-port body.