Abstract: Presented is an adaptor for use with a universal plotter pen that enables use of the universal plotter pen in many different X-Y plotter mechanisms. The interior configuration and dimensions of the adaptor, including elongated longitudinal ribs, are such as to snugly receive and frictionally retain the body of the plotter pen in a stable axial relationship with the adaptor body, which is tubular to receive the plotter pen body. A control boss is provided within the tubular body of the adaptor to limit the extent of insertion of the plotter pen, and slots are formed in the tubular body of the adaptor to give access to the exposed end of the plotter pen body that projects through the slots to facilitate digital imposition of an axially directed force on the end of the plotter pen to effect ejection of the plotter pen from the adaptor body.