Abstract: An endodontic instrument for treating a pulp canal includes a handle portion adjustably mounted in telescoping relation on the shaft of an elongated working tool, with cooperating grooves in the shaft and handle preventing relative movement therebetween during use of the instrument. Means are provided for quickly and accurately adjusting the working length of the tool projecting from the handle and for quickly and reliably interlocking the tool and handle in the desired adjusted relationship with the end of the handle acting as a positive stop limiting the depth of penetration of the working tool portion during use of the instrument.