It is well established that ionospheric convection patterns are strongly controlled by the interplanetary magnetic field (IMF). There has been an increased interest in ionospheric convection response to IMF changes. Some studies found that ionospheric flows change first near noon and later near the dawn-dusk meridian plane, which is interpreted as propagation or expansion of newly generated convection cells in the cusp region. Other studies showed that the change in convection pattern in response to IMF reorientations is spatially fixed. In this paper, we investigate the ionospheric convection response to IMF Bz changes during strong IMF By. On March 23, 1995, Bx was small, By was strongly positive (7--11 nT), and the Bz polarity changed several times after 1300 UT. The dayside ionospheric convection is dominated by a large clockwise convection cell. The cell focus (the eye of the convection pattern) is located in the prenoon sector for northward Bz and in the postnoon sector for southward Bz. It is found that the cell focus shifts from the prenoon sector to the postnoon sector following a southward Bz turning and vice versa for a northward Bz turning. However, the motion of the convection cell, or the largest change in the convection pattern, is limited roughly to the region between the previous cell focus and the new cell focus. Outside this region, the ionospheric flows could be greatly enhanced or weakened, while the convection pattern shape changes very little. When By is strong enough, the Bz reorientation causes changes in the flow intensity but not in the shape of the convection pattern. The results show the characteristics of ionospheric convection response during strong By and suggest that the convection reconfiguration is not only determined by the changing Bz but also significantly influenced by the stable and large By. ¿ 2000 American Geophysical Union