It is proposed that the Zeeman effect of the 2 02 → 1 11 transition in sulfur dioxide be used as a magnetic field calibration standard. The recent accurate beam maser measurements on this transition by Ellenbroek and Dymanus have been confirmed by electric resonance measurements in the 1 11 level. The Zeeman splittings most useful for calibration purposes depend linearly on the magnetic field. For perpendicular selection rules the Zeeman coefficient is 0.5282(3) MHz/kG while parallel transitions have a 0.3722(3) MHz/kG Zeeman coefficient. The sign of the rotational magnetic moment of SO 2 has been determined to be negative.