Abstract: A catheter for measuring the temperature of a fluid in a living body comprising an elongated tube having at least one lumen extending longitudinally within the tube and an opening in a peripheral wall of the tube. A thermistor mounting body mounts a thermistor in the lumen adjacent the opening. The thermistor mounting body defines a cavity at the opening which opens radially outwardly, and the thermistor is partially within the mounting body and projects from the mounting body into the cavity. There is at least one conductor in the tube with at least a region of the conductor being in the lumen and extending along the lumen to the thermistor. The region of the conductor is electrically coupled to the thermistor. The region of the conductor extends longitudinally of the lumen from a location in the lumen proximally of the thermistor to a location at which the conductor is electrically coupled to the thermistor.