An RF surgical resection instrument comprising - an asymmetric opening and closing resection loop for ensnaring and RF surgical separation of tissue, further comprising a short, electrically non-insulated loop section having a proximal and a distal end, and a long electrically insulated loop section having a proximal and a distal end, wherein the distal ends of the loop sections are mechanically and/or electrically interconnected, - a catheter made of electrically non-conductive material, having a proximal end and a distal end, and an opening in the longitudinal direction between the ends, - a manipulation wire in the catheter, having a proximal and a distal end, which can be advanced from proximal to distal inside the catheter in the longitudinal direction and can be withdrawn from distal to proximal, and its distal end is mechanically and/or electrically connected to the proximal end of the long loop section or which is formed as an extension of the proximal end of the long loop section, wherein the short loop section is pulled out from the distal end of the catheter during advance of the long loop section, - a stop device comprising a stop wire having a proximal and a distal end, wherein the distal end of the stop wire is connected to the proximal end of the short loop section, a first entrainer at the proximal end of the stop wire which is movable relative to the catheter, a stop which is fixed relative to the catheter, inside the catheter, on which the entrainer impacts when pulling out the short loop section so that the stop wire can be pulled out from the distal end of the catheter with a predefined length, and, - an entrainer device comprising the first entrainer and a second entrainer which is connected to the distal end of the manipulation wire, wherein upon withdrawal of the manipulation wire the second entrainer withdraws the first entrainer and therefore also draws the stop wire into the catheter characterized in that the stop wire has an electrical insulation at least on the section which can be pulled out from the catheter or over its entire length so that the insulation projects from the distal end of the catheter, when the first entrainer impacts on the stop. - an asymmetric opening and closing resection loop for ensnaring and RF surgical separation of tissue, further comprising a short, electrically non-insulated loop section having a proximal and a distal end, and a long electrically insulated loop section having a proximal and a distal end, wherein the distal ends of the loop sections are mechanically and/or electrically interconnected, - a catheter made of electrically non-conductive material, having a proximal end and a distal end, and an opening in the longitudinal direction between the ends, - a manipulation wire in the catheter, having a proximal and a distal end, which can be advanced from proximal to distal inside the catheter in the longitudinal direction and can be withdrawn from distal to proximal, and its distal end is mechanically and/or electrically connected to the proximal end of the long loop section or which is formed as an extension of the proximal end of the long loop section, wherein the short loop section is pulled out from the distal end of the catheter during advance of the long loop section, - a stop device comprising a stop wire having a proximal and a distal end, wherein the distal end of the stop wire is connected to the proximal end of the short loop section, a first entrainer at the proximal end of the stop wire which is movable relative to the catheter, a stop which is fixed relative to the catheter, inside the catheter, on which the entrainer impacts when pulling out the short loop section so that the stop wire can be pulled out from the distal end of the catheter with a predefined length, and, - an entrainer device comprising the first entrainer and a second entrainer which is connected to the distal end of the manipulation wire, wherein upon withdrawal of the manipulation wire the second entrainer withdraws the first entrainer and therefore also draws the stop wire into the catheter