As transition objects between the asymptotic giant branch (AGB) and cooling white dwarfs, the PG 1159 stars are key objects for understanding the late stages of stellar evolution. The pulsations exhibited by many members of the PG 1159 spectral class provide a mechanism for ferreting out this information. We present an analysis of the pulsation spectrum of the naked PG 1159 star PG 2131 +066, which we completely resolve using data from the Whole Earth Telescope (WET). The pulsation spectrum shows patterns that are expected from pulsation theory; comparison of these patterns with theoretical models indicate that PG 2131 has a mass of 0.61 ± 0.02 Mʘ, a luminosity of log (L/Lʘ) = 1.0 ± 0.2, and it lies ata distance of 470 +470 - 130pc. lt rotates with a period of 5.1 hr and shows evidence of a discontinuity in composition approximately 6 x 10- 3M* below the surface. Certain modes show complex structure in the power spectrum, while others show measur able frequency changes in 8 years. Fine structure in the prograde (m = -1) modes suggests the presence of nonlinear effects.