Strong enhancements of whistler mode hiss emissions have been observed to correlate with plasma density enhancements in the outer plasmasphere between L=4 and L=6. This indicates that these density enhancements are acting as whistler mode wave ducts. The wave and density observations were made simultaneously by the Aerospace swept frequency receiver aboard the AMPTE IRM spacecraft. The plasma density is determined from a narrow-band line near the plasma frequency. The hiss emissions generally occur below 2 kHz. The ducts consist of density enhancements of more than 40%. Density gradients on the sides of the ducts range from 0.10 to 0.24 el cm-3 km-1. The half width of each duct is typically 250 km. The wave intensity maximizes about 15 km from the center of the duct toward the outside, i.e., the side with a negative density gradient with increasing distance. The wave intensity near the center of a duct is an order of magnitude higher than the wave intensity at the density minimum between ducts. ¿ American Geophysical Union 1989