We present a unique Viking cusp observation during a period when the interplanetary magnetic field (IMF) observed by IMP 8 made a steplike change from southward to steady northward and subsequently back to steady southward again. The solar wind density and velocity were fairly steady during the entire period. Viking detected two independent plasma signatures. The first (equatorward part) is associated with southward IMF, and the second (poleward part) is associated with northward IMF. In the transition from the first signature to the second one, the ion data show an ''overlapping injection'' signature, i.e., two independent plasma populations exist on the same field lines and are well separated in the energy domain at any time and at any pitch angle. The second plasma population is found on the higher-energy side of the first population. A good relation between keV ion morphology and the field-aligned current pattern exists even though the particle cusp is undergoing a dynamic change. We discuss two possible scenarios for the present observation which involve the dynamic reaction of the cusp morphology to the stepwise change in the IMF condition. ¿ American Geophysical Union 1995