An endoscopically applicable instrument for RF surgical cutting and/or coagulation of biological tissues, and in particular for RF surgical circumferential cutting of polypoid or flat mucosal lesions of the gastrointestinal tract, comprising a flexible catheter having a distal end and a proximal end, which catheter can be moved through an instrumentation channel of a flexible endoscope, an effector with an RF surgical cutting and/or coagulation electrode at the distal end of the catheter, an RF power cable or connecting device for an RF power cable at the proximal end of the catheter, and an RF power supply line in the catheter which connects the RF surgical cutting and/or coagulation electrode with the RF power cable or with the connecting device for an RF power cable, characterized in that the effector comprises - a metallic support and/or sliding skid being electrically insulated with an insulating layer on its surface and having a distal and a proximal end, said support or sliding skid having an angling and/or bending between these two ends (4, 5), such that the distal end of the sliding skid or of the effector, respectively, has a distance (a) of the center axis of the distal end portion of the catheter, wherein the side of the effector facing away from the direction of the angling and/or bending is usable as a support and/or sliding surface, - an electrically non-insulated RF surgical cutting and/or coagulation electrode on the electrically insulated support and/or sliding surface being mechanically and electrically conductively connected with the metallic support and/or sliding skid, and thereby passes through the insulating layer, - an effector shaft at the proximal end of the metallic support and/or sliding skid, which effector shaft projects into the distal end of the catheter and is there mounted passively rotatable about the center axis of the distal end portion of the catheter, - a locking device for axially locking the effector shaft in the catheter in order to avoid unintentional axial movements of the effector shaft in the catheter, but without hindering the passive rotation of the effector shaft including the effector around the center axis, and - a rotationally flexible, electrically conductive connection between the metallic support and/or sliding skid including the RF surgical cutting and/or coagulation electrode and an RF power cable or a device for connecting an RF power cable to the proximal end of the catheter. - a metallic support and/or sliding skid being electrically insulated with an insulating layer on its surface and having a distal and a proximal end, said support or sliding skid having an angling and/or bending between these two ends (4, 5), such that the distal end of the sliding skid or of the effector, respectively, has a distance (a) of the center axis of the distal end portion of the catheter, wherein the side of the effector facing away from the direction of the angling and/or bending is usable as a support and/or sliding surface, - an electrically non-insulated RF surgical cutting and/or coagulation electrode on the electrically insulated support and/or sliding surface being mechanically and electrically conductively connected with the metallic support and/or sliding skid, and thereby passes through the insulating layer, - an effector shaft at the proximal end of the metallic support and/or sliding skid, which effector shaft projects into the distal end of the catheter and is there mounted passively rotatable about the center axis of the distal end portion of the catheter, - a locking device for axially locking the effector shaft in the catheter in order to avoid unintentional axial movements of the effector shaft in the catheter, but without hindering the passive rotation of the effector shaft including the effector around the center axis, and - a rotationally flexible, electrically conductive connection between the metallic support and/or sliding skid including the RF surgical cutting and/or coagulation electrode and an RF power cable or a device for connecting an RF power cable to the proximal end of the catheter.