Abstract: Methods for inserting a bundle of hollow fiber membranes into a permeator shell are provided which methods present minimal risk of damage to the hollow fiber membranes and enable high packing factors of hollow fiber membranes within the permeator shell to be achieved. In the methods, a bundle of hollow fiber membranes is surrounded by a substantially fluid impermeable, flexible envelope. The interior of the envelope is provided at a lower total pressure than the pressure at the exterior of the envelope such that the flexible envelope is drawn into contact with the bundle and laterally compacts the bundle. The envelope containing the bundle is inserted into a permeator shell while maintaining the pressure differential. The flexible envelope may be retained within or may be withdrawn from the permeator shell.