A pliers-type insulation stripping tool includes a releasable fastening device for fastening a knife cassette to at least one of the knife carrier members that are longitudinally displaced by the pivotally connected tool handle members to remove a severed length of insulation from a longitudinally-arranged insulated conductor that is clamped between by the tool gripping jaws. A U-shaped resilient fastener spring wire has leg portions that are resiliently biased apart, at least one of the legs containing a laterally extending bulge portion that extends into a corresponding locking slot contained in a side wall of the knife cassette. To release and replace the knife cassette, the protruding free extremities of the leg portions of the spring wire are displaced together, thereby to remove the leg bulge portion from the slot contained in the cassette outer wall.