A mathematical model is developed for the film-mounted thermistor bead temperature sensor used in current meteorological rocketsondes. Thermometric correction formulas are given suited to programming for automatic data processing. The development treats the convection, radiation, electric heating and dynamic lag of the thermistor bead, along with the convective, radiative and conductive behavior of its supporting electric wires and film. The effects are combined according to conductive coupling. Basic expressions are included for special cases of long, short and insulated wires, and analogous cases in the film. The model is generally applicable when the wire and film are sufficiently thin. A numerical example based on the White Sands Missile Range STS-1 sensor establishes the film as the dominant element in sensing air temperatures above the stratopause.