Forceps instrument comprising a sheath having a first end and an elongated maneuvering element slidably disposed in the first end of said sheath. The sheath has an axis. The instrument comprises two jaw-pieces, at least one of the jaw-pieces being movable away from and toward the other of the jaw-pieces when the maneuvering element slides axially in the first end of the sheath. The jaw-pieces have, in an extension of a part thereof, an arm extending progressively toward the axis of the sheath so as to form a first, internal, inclined surface extending progressively toward the axis and facing the latter and a second, external, inclined surface extending progressively toward the axis and facing the internal surface of the sheath. The arm has a convex excrescence which is slidable substantially against the internal surface of the sheath, the excrescence having a concave hollow cavity facing the axis. The sheath has, near its end beyond which the jaws extend, a transverse element against which the internal surface of the arm slides. The elongate maneuvering element terminates in a head housed in the concave hollow cavity so that, when the elongate maneuvering element is moved toward a free end of the sheath, it pushes the jaw-pieces back beyond the end of the sheath, thus causing the internal surface of the arm, which slides over the transverse element, to pivot by a ramp effect, moving the at least one jaw of the jaw-pieces away from the other jaw, the movement away being permitted by the inclination of the external surface which protrudes from the end of the internal surface of the sheath. The excrescence of the arm pivoting, during this movement, about the head while still being guided in the sheath, reverse movement of the maneuvering element causing the jaw-pieces to move closer by a reverse movement. The transverse element includes a passage having an axis substantially parallel to the axis of the sheath for accommodating an auxiliary tool between the two jaw-pieces.