Analysis of all the existing magnetic field data from Saturn's magnetosphere is carried out. These data arise from the three spacecraft encounters with Saturn (Pioneer 11, Voyager 1 and 2). In order to carry out a direct comparison between the three magnetic field data sets, they are all expressed in the same planetocentric spherical polar coordinate system. In the cases of Pioneer 11 (whole encounter) and Voyager 2 (inbound) a surprising perturbation of a few nanoteslas is observed (with a period close to Saturn's rotation period) in the radial and/or azimuthal components of the magnetic field. It is shown that a dipole tilt and/or current sheet crossings are unlikely causes for such periodic perturbations. This is in agreement with the negligible magnetic dipole tilt resulting from the existing models of Saturn's magnetic field. ¿ 2000 American Geophysical Union