A 3-D global fluid simulation is used to investigate the changing magnetic field topology of the magnetotail as observed by Geotail and IMP 8. The event studied is of particular interest as the solar wind density and speed as observed by WIND were approximately constant so that the influence of the interplanetary magnetic field (IMF) can be isolated. Loading of the tail fields during southward IMF is seen at high latitudes with IMP 8 moving from the sheath into the magnetosphere while at low latitudes Geotail moves from the plasma sheet into the lobes. The reverse is true for northward turnings. The tail cross-section is shown to be elliptical during southward IMF with an eccentricity of about 0.2 and that this eccentricity is slowly eroded over a period of about an hour during northward IMF.¿ 1997 American Geophysical Union