An improved infusion device is of the type which can be implanted for drug delivery within a patient's body, and supplied with infusate without the need for surgical removal. The infusion device has a housing forming a chamber and an entry port into the chamber. The entry port is closed by a septum. The septum has a convex top section extending from the housing, and a frusto-conical section which matingly engages a similarly shaped interior surface of the housing. The interior surface can be provided with a plurality of notches with which the septum engages by material flexure. This arrangement improves retention of the septum and operation of the infusion device, and permits advances in the method by which such devices are manufactured.