Experimental observations of the seismic efficiency of a decoupled nuclear explosion in a salt dome are presented. The energy decoupling factor was calculated from local, regional, and teleseismic data using the ratio of the amplitudes of the tamped (well coupled) explosion that had created the cavity and that of the decoupled one. To calculate the amplitudes of the seismic waves of a well coupled explosion of the same yield as the decoupled one, a scaling law for explosions fired within the same region was used. An energy decoupling factor near 30 is consistent with local, regional, and teleseismic data. It is concluded that decoupling was only partial for the event described. Numerical calculations of partial decoupling showed that the seismic wave spectrum at an epicentral distance of 2.3 km is consistent with an amplitude decoupling factor of 20, which is consistent with the energy decoupling factor 30. ¿ American Geophysical Union 1993