A utility knife having a handle and a blade holder which is pivotally mounted on the handle for movement from an unfolded position to a folded position. The handle has a space adapted to receive the blade holder when the blade holder is in its folded position. A spring pressed lock plug is provided to move into and out of the space to hold the blade holder in the folded and unfolded positions. The blade holder includes a main wall and guard wall pivotally mounted on the main wall to pivot from an open position to a closed position overlying the main wall. The main wall being adapted to hold a blade so that a blade is interposed between the main wall and guard wall when the guard wall is in its closed position.