Abstract: An automatically retractable slitter for cutting thermoplastic sheet as it exits from the sheet die of an extruder comprising a housing, a cutter blade mounted in a holder, a recess in the housing of a length greater than the length of the holder for slidably and retractably receiving the holder and blade, an elongated slot in said housing having detent means, an elongated pin secured to said holder and extending transversely through the elongated slot, spring means between the holder and the housing adapted to be relaxed when the holder is in its retractable position within the recess, the pin being adapted to rest in the detent when a cutting force is applied to the blade and to disengage from the detent when no force is applied to the blade.