Unfossiliferous pelagic clay (red clay) cores from the Pacific generally have unstable remanent magnetization except for tens of centimeters to several meters below the surface. The origin of this instability has been considered to be viscous remanent magnetization (VRM). We conducted a rock-magnetic study of a pelagic clay core from the South Pacific, which has the unstable-to-stable transition at about 1.5 m below the surface. Using the suspension method of Yoshida and Katsura (1985), we determined downcore variation of complete alignment magnetization (CAM) and magnetic moment distribution (geometric mean moment, mG, and log standard deviation, log a) assuming a lognormal distribution. Mean magnetic grain diameter was calculated from the mG using the saturation magnetization of magnetite, 4.8¿105 A m-1, on the assumption that all the magnetic grains are single-domain magnetites of spherical shape. This assumption is supported by thermomagnetic analyses, isothermal remanent magnetization (IRM) acquisition experiments, and the ratio of CAM to saturation IRM. The grain diameter ranges from about 0.02 to 0.15 μm and decreases with depth below about 1.5 m. The derived grain size holds even when the magnetites in the sediments suffered oxidation because the saturation magnetization of maghemite is close to that of magnetite. The frequency dependence of magnetic susceptibility showed a downward increase in the amount of superparamagnetic grains. On the other hand, the magnetic viscosity acquisition coefficient normalized by the CAM increases with depth. We conclude that magnetic grain size can control the magnitude of the secondary magnetization, VRM, of pelagic clay. It has been proposed that the major source of pelagic clay is atmospherically transported dust. Intensified global atmospheric circulation by the change of climate would raise the grain size of the magnetic minerals of the eolian component, which can cause the unstable-to-stable transition of the remanent magnetization. ¿ American Geophysical Union 1990