  The star zeta Ophiuchi is one of the brightest massive stars in the northern
hemisphere and was intensively studied in various wavelength domains. The
currently available observational material suggests that certain observed
phenomena are related to the presence of a magnetic field. We acquired
spectropolarimetric observations of zeta Oph with FORS1 mounted on the 8-m
Kueyen telescope of the VLT to investigate if a magnetic field is indeed
present in this star. Using all available absorption lines, we detect a mean
longitudinal magnetic field B_z_all = 141+-45G, confirming the magnetic nature
of this star. We review the X-ray properties of zeta Oph with the aim to
understand whether the X-ray emission of zeta Oph is dominated by magnetic or
by wind instability processes.
