Zeeman spectroscopic observations of the helium variable a Cen suggest that a magnetic field, varying between + 1000 and - 1000 gauss, is present in this star. Magnetic maximum coincides in phase with helium minimum. The radial velocity variations of the lines of He I are consistent with the hypothesis that the helium/hydrogen ratio varies over the surface of a Cen and that the helium spectrum variability is produced by rotation of this inhomogeneous surface. Key words: helium variable - magnetic field - radial velocity