A device for determining at least one parameter of a medium which flows in a conduit has a fluid conduit part having a wall which forms a fluid conduit passage and has an insertion opening, a plug sensor having a sensor element, an insertion portion which is inserted through the insertion opening of the wall into the fluid conduit passage, an electrical connection portion which is not inserted into the fluid conduit passage, and a stop surface which abuts against a stop surface of the fluid conduit part, and mounting means for mounting the plug sensor on the fluid conduit part, the mounting means including a holding part which is pushed substantially perpendicular to an insertion direction of the plug sensor onto a connecting pipe of the fluid conduit part which surrounds the insertion opening, the holding part having at least one first portion abutting against a supporting surface of the connecting pipe and at least one second portion applying a pretensioning force to a securing surface of the plug sensor which faces away from the insertion portion of the plug sensor, so that a stop surface of the plug sensor abuts against the stop surface of the fluid conduit part.