Some volcanic tremor has the extreme persistence of oscillation that may be attributed to resonance. Our theoretical analysis suggests that a spherical resonator consisting of volcanic fluid has a low attenuation mode (LAM) at some of the lowest frequencies, which can be as low as 0.01 times the attenuation factors of higher modes in some cases. The monotonic waveforms of volcanic tremor (LP-events) at Asama Volcano, Japan, can be explained by the LAM excitation in the spherical resonator model, which well fits both characteristic frequencies and attenuation factors. The resonator size, velocity, and the density contrasts between the fluid inside and the elastic medium outside are estimated. ¿ 1999 American Geophysical Union