The discovery of linear polarization of the radio emission from the pulsating radio stars1 provides an opportunity for the first direct measurement of the average magnetic field over distances of up to 100 pc from the Sun. It has already been shown2,3 that the frequency dispersion in arrival time of the pulses can be entirely explained by a dispersion in group velocity in ionized interstellar gas, and for the pulsating star CP 1919 the integrated electron density ∫N dl along the line of sight has been found3 to be 12·55 cm-3 pc. If a magnetic field has a component H 11 along the line of sight, then the Faraday rotation of the plane of polarization is a measure of ∫N H 11 dl; the ratio of the two integrals then gives H 11, the average value of H 11 along the line of sight, weighted according to the electron density.