We study a Pi2 pulsation that occurred at ~1520 UT on 29 August 2000. This Pi2 event was observed at ground stations from high (geomagnetic latitude ~65¿) to low latitudes (~17¿) near midnight with an identical waveform and oscillated with a frequency of ~11 mHz. During the event, a global image of the plasmasphere was obtained from the IMAGE satellite, and the location of the plasmapause was clearly identified. The plasmasphere was small and strongly asymmetric in longitude. The plasmapause was located at L ~ 2.4, 3.8, and 3.3 near the duskside, midnight, and the dawnside, respectively. Using a magnetospheric mass density model constructed from the IMAGE satellite data and ground-based data, we examine whether the Pi2 pulsation observed inside the plasmasphere can be explained by a plasmaspheric cavity mode. We find that the frequency of 11 mHz is too low for a cavity mode in the plasmasphere. Thus the plasmaspheric cavity mode is not an appropriate model for our Pi2 event observed at midlatitudes and low latitudes. We discuss what determines its period and waveform inside such a small and asymmetric plasmasphere.