An instrument (10) for fusing and severing vessels in accordance with the invention comprises a first branch (19) with a first electrode (33) and a second branch 20 that is supported so as to be movable toward and away from the first electrode (33), wherein the second branch (20) has a longitudinal slit (31). A knife (21) is arranged in said slit so as to be movable longitudinally relative to the shaft (11). The knife (21) is associated with a vertical adjustment mechanism (54) for adjusting the knife (21) perpendicularly to the longitudinal movement as a function of the longitudinal movement of said knife. As a result of this, a premature dulling of the tip (28b) or the sliding edge (28a) of the knife 21 due to contact with the ceramic base body (23) or the edges or the step (55) is avoided.
