An instrument that can be used endoscopically for HF surgical cutting and/or coagulation of biological tissues, and in particular for HF surgical excision of polyp-like or flat lesions of the mucosa of the gastrointestinal tract, comprises a flexible catheter with a distal end and a proximal end, which catheter can be moved through an instrument channel of a flexible endoscope, and an effector with an HF surgical cutting and/or coagulating electrode at the distal end of the catheter. The effector comprises a metallic supporting and/or sliding runner which is electrically insulated on its surface with an insulating layer and which has a distal end and a proximal end and, between these two ends, is angled and/or bent in such a way that the distal end of the sliding runner or of the effector is at a distance from the central axis of the distal end portion of the catheter, wherein the side of the effector directed away from the direction of the angle and/or bend can be used as a supporting and/or sliding surface.
