This paper reports a large southward magnetic field of -23.5 nT in the Earth's magnetotail at a radial distance of 46 RE, observed with Geotail on January 10, 1995. The large southward field was part of a bipolar Bz signature of a plasmoid in association with a well-isolated substorm under steady solar wind conditions. Magnetic field and plasma observations are analyzed and compared with results from a hybrid simulation for magnetic reconnection. The large southward field is produced by the piling up of field lines due to colliding fast tailward flowing plasmas. This study implies that the piling-up mechanism plays an important role in the evolution of plasmoids traveling in the magnetotail. ¿ 1998 American Geophysical Union