Abstract: A retractable writing instrument has a push-to-extend, push-to-retract operating mechanism assembled for life as a single module which is removeable from a body of the writing instrument. A prehensile member is connected to the operating mechanism and is moveable axially for displacing the writing tip of the cartridge between its extended and retracted positions. The prehensile member has resilient parts adapted for grasping the end of the cartridge inserted through an aperture in the operating mechanism module and pressed into a cavity in the prehensile member, thereby allowing users to readily replace the cartridge without the risk of releasing small easily-lost components of the operating mechanism.