A system for positioning a sonde in a cylindrical hollow tube of a borehole, said tube being defined along a longitudinal axis and including longitudinal slots, the sonde being suitable for measuring a parameter of a medium in which said borehole is formed, said positioning needing to be performed from the proximal end of the tube, the system being characterized in that it comprises: • a groove formed in the inside wall of the tube, the groove having a shape defined both by a first substantially frustoconical surface having minor and major bases (33 and 34) defined respectively in two planes substantially perpendicular to the longitudinal axis of the tube, the minor base coinciding with an inside cross-section of the tube and being situated between the proximal end of the tube and the major base of the first frustoconical surface, and also by an annular surface defined in the plane of the major base of the first frustoconical surface, said annular surface being bordered, on its outside edge, by said first frustoconical surface, and on its inside edge, by the inside wall of the tube; • a support, the support being arranged so as to be suitable for moving in translation in the tube along said longitudinal axis; • means for mounting the sonde and the support in mutual co-operation; • an open cylindrical housing arranged in the support, opening out facing the inside wall of the hollow tube, and defined along an axis substantially perpendicular to said longitudinal axis when the support is moving in translation in the tube; and • at least one first bolt mounted to move in translation in said housing against a resilient force, said first bolt being suitable for occupying all positions in said housing between two extreme positions, a "retracted" first position and an "extended" second position defined so that its end facing towards the inside wall of the tube and suitable for emerging from the housing penetrates into said groove under the action of the resilient force when the bolt comes to face the groove, until it comes into abutment against at least a portion of said annular surface, said second position being a stable position; and that • said means for mounting said probe and said support in mutual co-operation are arranged in such a manner that when said bolt is positioned inside groove, said sonde is substantially facing the longitudinal slots. • a groove formed in the inside wall of the tube, the groove having a shape defined both by a first substantially frustoconical surface having minor and major bases (33 and 34) defined respectively in two planes substantially perpendicular to the longitudinal axis of the tube, the minor base coinciding with an inside cross-section of the tube and being situated between the proximal end of the tube and the major base of the first frustoconical surface, and also by an annular surface defined in the plane of the major base of the first frustoconical surface, said annular surface being bordered, on its outside edge, by said first frustoconical surface, and on its inside edge, by the inside wall of the tube; • a support, the support being arranged so as to be suitable for moving in translation in the tube along said longitudinal axis; • means for mounting the sonde and the support in mutual co-operation; • an open cylindrical housing arranged in the support, opening out facing the inside wall of the hollow tube, and defined along an axis substantially perpendicular to said longitudinal axis when the support is moving in translation in the tube; and • at least one first bolt mounted to move in translation in said housing against a resilient force, said first bolt being suitable for occupying all positions in said housing between two extreme positions, a "retracted" first position and an "extended" second position defined so that its end facing towards the inside wall of the tube and suitable for emerging from the housing penetrates into said groove under the action of the resilient force when the bolt comes to face the groove, until it comes into abutment against at least a portion of said annular surface, said second position being a stable position; and that • said means for mounting said probe and said support in mutual co-operation are arranged in such a manner that when said bolt is positioned inside groove, said sonde is substantially facing the longitudinal slots.